Workaround 'GC_pthread_*, begin/endthreadex never used' cppcheck warnings

Workaround 'GC_pthread_*, begin/endthreadex never used' cppcheck warnings
(fix commit c5d4ef9)

* tests/test.c [CPPCHECK] (UNTESTED): Move outside main().
* tests/test.c [!PCR && !GC_WIN32_THREADS && !GC_PTHREADS && CPPCHECK]
(main): Remove UNTESTED(test).
* tests/test.c [!PCR && !GC_WIN32_THREADS && !GC_PTHREADS && CPPCHECK
&& GC_PTHREADS] (main): Move UNTESTED(GC_pthread_detach),
UNTESTED(GC_set_suspend_signal), UNTESTED(GC_set_thr_restart_signal),
UNTESTED(GC_dlopen), UNTESTED(GC_pthread_cancel),
UNTESTED(GC_pthread_exit), UNTESTED(GC_pthread_sigmask) to main() which
is defined if GC_PTHREADS.
* tests/test.c [!PCR && !GC_WIN32_THREADS && !GC_PTHREADS && CPPCHECK
&& GC_WIN32_THREADS] (main): Move UNTESTED(GC_ExitThread),
UNTESTED(GC_beginthreadex), UNTESTED(GC_endthreadex) to main() which
is defined if GC_WIN32_THREADS.
* tests/test.c [PCR] (test): Call GC_noop1((word)&test) if CPPCHECK.