Avoid use of os.mknod() in Python testing for portability

[samsrabin]

It doesn't work everywhere; e.g., on my Mac. Instead, use the safer os.makedirs() for making empty directories or pathlib.Path.touch() for empty files.

Unit tests failing on my Mac because of this:

• test_append_user_nl_step2: PermissionError at os.mknod(ufile)
• test_run_phase: PermissionError in SSPCaseFake.create_clone() at os.mknod(os.path.join(newcase, "user_nl_clm"))