tests: Avoid GCC over-optimization caused by _GL_ARG_NONNULL attributes.
Reported by Jim Meyering in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-01/msg00040.html>.
* lib/stdlib.in.h (GNULIB_defined_canonicalize_file_name): New macro.
(GNULIB_defined_ptsname_r): New macro.
* tests/test-canonicalize.c (_GL_ARG_NONNULL): Define to empty.
(main): Disable the NULL argument test if canonicalize_file_name does
not come from gnulib.
* tests/test-canonicalize-lgpl.c (_GL_ARG_NONNULL): Define to empty.
(main): Disable the NULL argument test if canonicalize_file_name does
not come from gnulib.
* tests/test-ptsname_r.c (_GL_ARG_NONNULL): Define to empty.
(test_errors): Disable the NULL argument test if ptsname_r does not come
from gnulib.