tests: Avoid some more runtime errors with Fil-C.

tests: Avoid some more runtime errors with Fil-C.

* tests/test-c32ispunct.c (main): Skip a test case that fails on Fil-C.
* tests/test-free.c (main): Avoid converting from a pointer to uintptr_t
and back.
* tests/test-explicit_bzero.c (test_heap): Skip this test on Fil-C.
* tests/test-memset_explicit.c (test_heap): Likewise.
* tests/test-sigsegv-catch-segv1.c: Skip the entire test on Fil-C, since
it relies on converting an uintptr_t value to a pointer.
* tests/test-sigsegv-catch-segv2.c: Likewise.