tests: delete libhostname.so and chkhostname

Before this patch, `libhostname.so` and `chkhostname` were a test
facility for overriding `gethostname()` in non-debug builds on
Linux and other Unix platforms supporting `LD_PRELOAD`.

`gethostname()` has a single use with SMTP.

The alternative way to override `gethostname()` is building in debug
mode, which allows to do this via the `CURL_GETHOSTNAME` env, on all
platforms.

Drop the `LD_PRELOAD` solution in favour of the above.

Also:
- delete inactive NTLM code with a `gethostname()` call made from it.
- streamline NTLM code by dropping a `printf()` and a macro.
- tests: stop setting `CURL_GETHOSTNAME` where unnecessary.

Closes #14695

tests/libtest/Makefile.am

@@ -63,14 +63,7 @@ LDADD = $(SUPPORTFILES_LIBS)
 # noinst_PROGRAMS, lib*_SOURCES, and lib*_CFLAGS)
 include Makefile.inc
 
-# Preloading of libhostname allows host name overriding,
-# this is used to make some tests machine independent.
-
-if BUILD_LIBHOSTNAME
-noinst_LTLIBRARIES = libhostname.la
-else
 noinst_LTLIBRARIES =
-endif
 
 if USE_CPPFLAG_CURL_STATICLIB
 AM_CPPFLAGS += -DCURL_STATICLIB
@@ -79,35 +72,12 @@ endif
 AM_LDFLAGS =
 AM_CFLAGS =
 
-libhostname_la_CPPFLAGS_EXTRA =
-libhostname_la_LDFLAGS_EXTRA = -module -avoid-version -rpath /nowhere
-libhostname_la_CFLAGS_EXTRA =
-
 libstubgss_la_LDFLAGS_EXTRA =
 
 if CURL_LT_SHLIB_USE_NO_UNDEFINED
-libhostname_la_LDFLAGS_EXTRA += -no-undefined
 libstubgss_la_LDFLAGS_EXTRA += -no-undefined
 endif
 
-if CURL_LT_SHLIB_USE_MIMPURE_TEXT
-libhostname_la_LDFLAGS_EXTRA += -mimpure-text
-endif
-
-if DOING_CURL_SYMBOL_HIDING
-libhostname_la_CPPFLAGS_EXTRA += -DCURL_HIDDEN_SYMBOLS
-libhostname_la_CFLAGS_EXTRA += $(CFLAG_CURL_SYMBOL_HIDING)
-endif
-
-libhostname_la_CPPFLAGS = $(AM_CPPFLAGS) $(libhostname_la_CPPFLAGS_EXTRA)
-libhostname_la_LDFLAGS = $(AM_LDFLAGS) $(libhostname_la_LDFLAGS_EXTRA)
-libhostname_la_CFLAGS = $(AM_CFLAGS) $(libhostname_la_CFLAGS_EXTRA)
-
-libhostname_la_SOURCES = sethostname.c
-
-libhostname_la_LIBADD =
-libhostname_la_DEPENDENCIES =
-
 # Build a stub gssapi implementation for testing
 if BUILD_STUB_GSS
 noinst_LTLIBRARIES += libstubgss.la