test-skeleton: Support temporary files without memory leaks [BZ#18333]

add_temp_file now makes a copy which is freed by delete_temp_files.
Callers to create_temp_file can now free the returned file name to
avoid the memory leak.  These changes do not affect the leak behavior
of existing code.

Also address a NULL pointer derefence in tzset after a memoru allocation
failure, found during testing.