tests: fix UTF-8 detection, per-test LC_* settings, CI coverage

- runtests: fix `codeset-utf8` feature detection. Before this patch it
  detected if the calling environment had UTF-8 enabled. If not, UTF-8
  tests were all skipped. After this patch, it detects if UTF-8 is
  supported by the calling environment regardless of what's currently
  enabled.
  Follow-up to 0b70b23ef4 #15039

- GHA/linux: sync `codeset-test` to also reset `LC_CTYPE` and
  `LC_NUMBER`. To give it more spin.
  Follow-up to c221c0ee59 #17938

- GHA/macos: fix to actually enable `codeset-test`. Also set `LC_ALL`,
  which seems necessary to trigger issues.
  Follow-up to c221c0ee59 #17938

- tests/data: replace `LC_CTYPE` env with `LC_ALL` in all tests
  requiring a locale. Also to avoid potential issues with a blank or
  unset `LC_ALL`, as seen earlier. And to ensure that the override works
  on all platforms (as tested in CI.)
  Slight downside is that this now resets the language/culture to `C`.
  Ref: b4c9982382 #4743
  Ref: 23208e330a #4738

- replace `en_US.UTF-8` with `C.UTF-8` to be language/culture-agnostic.

- TEST-SUITE.md: drop `UTF-8` as a requirement for tests.
  Tests shall work (or least be skipped) without UTF-8 support.

Tests requiring UTF-8 locale:
165, 962, 963, 964, 965, 966, 967, 1448, 1560, 2046, 2047
Tests requiring UTF-8 locale, but passing without one anyway:
955, 956, 957, 958, 959, 960, 961, 968, 1034, 1035

Spec 1997: https://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html
Spec 2008: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html

Ref: c221c0ee59 #17938
Ref: 7cf8414fab
Ref: 4c140a5628
Ref: 28faaacee2 #2436
Ref: ecd1d020ab

Closes #17988

tests/data/test1034

@@ -29,8 +29,7 @@ proxy
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 HTTP over proxy with malformatted IDN host name

tests/data/test1035

@@ -27,8 +27,7 @@ proxy
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 HTTP over proxy with too long IDN host name

tests/data/test1448

@@ -43,8 +43,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 Redirect following to UTF-8 IDN host name

tests/data/test1560

@@ -13,7 +13,7 @@ urlapi
 none
 </server>
 <setenv>
-LC_ALL=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <features>
 file

tests/data/test165

@@ -33,8 +33,7 @@ proxy
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 HTTP over proxy with IDN host name

tests/data/test2046

@@ -43,8 +43,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 Connection reuse with IDN host name

tests/data/test2047

@@ -44,8 +44,7 @@ proxy
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 Connection reuse with IDN host name over HTTP proxy

tests/data/test955

@@ -24,8 +24,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP without SMTPUTF8 support - UTF-8 based sender (local part only)

tests/data/test956

@@ -21,8 +21,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP without SMTPUTF8 support - UTF-8 based recipient (local part only)

tests/data/test957

@@ -22,8 +22,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP VRFY without SMTPUTF8 support - UTF-8 recipient (local part only)

tests/data/test958

@@ -22,8 +22,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP external VRFY without SMTPUTF8 - UTF-8 recipient (local part only)

tests/data/test959

@@ -25,8 +25,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP without SMTPUTF8 support - UTF-8 based sender (host part only)

tests/data/test960

@@ -22,8 +22,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP without SMTPUTF8 support - UTF-8 based recipient (host part only)

tests/data/test961

@@ -23,8 +23,7 @@ smtp
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP external VRFY without SMTPUTF8 - UTF-8 recipient (host part only)

tests/data/test962

@@ -23,8 +23,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP without SMTPUTF8 support - UTF-8 based sender (host part only)

tests/data/test963

@@ -23,8 +23,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP without SMTPUTF8 support (IDN) - UTF-8 recipient (host part only)

tests/data/test964

@@ -24,8 +24,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP external VRFY without SMTPUTF8 (IDN) - UTF-8 recipient (host part)

tests/data/test965

@@ -26,8 +26,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP with SMTPUTF8 support - UTF-8 based sender

tests/data/test966

@@ -26,8 +26,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP with SMTPUTF8 support - UTF-8 based recipient

tests/data/test967

@@ -30,8 +30,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP external VRFY with SMTPUTF8 support

tests/data/test968

@@ -27,8 +27,7 @@ IDN
 codeset-utf8
 </features>
 <setenv>
-LC_ALL=en_US.UTF-8
-LC_CTYPE=en_US.UTF-8
+LC_ALL=C.UTF-8
 </setenv>
 <name>
 SMTP VRFY with SMTPUTF8 support