tests: unset some envs instead of blanking them

After 7cf8414fab #12862, `VAR=` no longer
removes the env variable, but sets it to an empty/blank value instead.
To remove an env, `VAR` shall be used (without the assigment operator.)

`SSL_CERT_FILE`, `CURL_HOME`, `HOME`, `XDG_CONFIG_HOME`, were added
before the change above. Make tests unset these envs again, as their
commit messages suggest, instead of blanking them. It does not change
the outcome of the tests.

Ref: 764e4f066d #8213
Ref: e992770e8d #6600

Folllow-up to 7cf8414fab #12862
Cherry-picked from #17988
Closes #17994

tests/data/test1400

@@ -31,7 +31,7 @@ http
 --libcurl for simple HTTP GET
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://%HOSTIP:%HTTPPORT/we/want/%TESTNUMBER --libcurl %LOGDIR/test%TESTNUMBER.c

tests/data/test1401

@@ -39,7 +39,7 @@ cookies
 --libcurl
 </features>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://%HOSTIP:%HTTPPORT/we/want/%TESTNUMBER --libcurl %LOGDIR/test%TESTNUMBER.c --basic -u fake:user -H "X-Files: Mulder" -H "X-Men: cyclops, iceman" -A MyUA -b chocolate=chip --proto "=http,ftp,file"

tests/data/test1402

@@ -32,7 +32,7 @@ http
 --libcurl for simple POST
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://%HOSTIP:%HTTPPORT/we/want/%TESTNUMBER --libcurl %LOGDIR/test%TESTNUMBER.c -d "foo=bar" -d "baz=quux"

tests/data/test1403

@@ -32,7 +32,7 @@ http
 --libcurl for GET with query
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://%HOSTIP:%HTTPPORT/we/want/%TESTNUMBER --libcurl %LOGDIR/test%TESTNUMBER.c -G -d "foo=bar" -d "baz=quux"

tests/data/test1404

@@ -34,7 +34,7 @@ http
 --libcurl plus -F with 3 files, one with explicit type & encoder
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://%HOSTIP:%HTTPPORT/we/want/%TESTNUMBER -F name=value -F 'file=@%LOGDIR/test%TESTNUMBER.txt,%LOGDIR/test%TESTNUMBER.txt;type=magic/content;encoder=8bit,%LOGDIR/test%TESTNUMBER.txt;headers=X-testheader-1: header 1;headers=X-testheader-2: header 2' --libcurl %LOGDIR/test%TESTNUMBER.c

tests/data/test1405

@@ -36,7 +36,7 @@ ftp
 --libcurl for FTP with quote ops
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 ftp://%HOSTIP:%FTPPORT/%TESTNUMBER -Q "NOOP 1" -Q "+NOOP 2" -Q "-NOOP 3" -Q "*FAIL" -Q "+*FAIL HARD" --libcurl %LOGDIR/test%TESTNUMBER.c

tests/data/test1406

@@ -26,7 +26,7 @@ smtp
 --libcurl for SMTP
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <file name="%LOGDIR/test%TESTNUMBER.eml">
 From: different

tests/data/test1407

@@ -27,7 +27,7 @@ pop3
 --libcurl for POP3 LIST one message
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 pop3://%HOSTIP:%POP3PORT/%TESTNUMBER -l -u user:secret --libcurl %LOGDIR/test%TESTNUMBER.c

tests/data/test1420

@@ -33,7 +33,7 @@ imap
 --libcurl for IMAP FETCH message
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 'imap://%HOSTIP:%IMAPPORT/%TESTNUMBER/;MAILINDEX=1' -u user:secret --libcurl %LOGDIR/test%TESTNUMBER.c

tests/data/test1465

@@ -29,7 +29,7 @@ http
 --libcurl for POST with binary content
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://%HOSTIP:%HTTPPORT/we/want/%TESTNUMBER --libcurl %LOGDIR/test%TESTNUMBER.c --data-binary @%LOGDIR/%TESTNUMBER-upload

tests/data/test1481

@@ -33,7 +33,7 @@ SSL
 --libcurl with TLS version options
 </name>
 <setenv>
-SSL_CERT_FILE=
+SSL_CERT_FILE
 </setenv>
 <command>
 http://moo/ --libcurl %LOGDIR/test%TESTNUMBER.c --tls-max 1.3 --proxy-tlsv1 -x http://%HOSTIP:%HTTPPORT

tests/data/test433

@@ -30,8 +30,8 @@ http
 </server>
 <setenv>
 XDG_CONFIG_HOME=%PWD/%LOGDIR
-HOME=
-CURL_HOME=
+HOME
+CURL_HOME
 </setenv>
 <name>
 Verify XDG_CONFIG_HOME use to find curlrc

tests/data/test436

@@ -30,7 +30,7 @@ http
 </server>
 <setenv>
 CURL_HOME=%PWD/%LOGDIR
-XDG_CONFIG_HOME=
+XDG_CONFIG_HOME
 </setenv>
 <name>
 Find .curlrc in .config/curlrc via CURL_HOME