Allow to force executable pages allocation in gctest

Allow to force executable pages allocation in gctest

Pass -D TEST_PAGES_EXECUTABLE to CFLAGS to enable the executable pages
allocation in gctest even if the collector is compiled with
NO_EXECUTE_PERMISSION.

* tests/test.c (INIT_PAGES_EXECUTABLE): New macro (calls
GC_set_pages_executable(1) if TEST_PAGES_EXECUTABLE).
* tests/test.c (GC_COND_INIT): Call INIT_PAGES_EXECUTABLE (before
GC_OPT_INIT).
* tests/test.c [!PCR && !GC_WIN32_THREADS && !GC_PTHREADS && CPPCHECK]
(main): Remove UNTESTED(GC_set_pages_executable).