fusefs: fix multiple issues with the io tests

• During TearDown, close the test file before the backing file. That way the backing file artifact will have the correct contents after the test completes. It doesn't matter when running in Kyua, but it may when running the test manually.
• Add a closeopen operation that mimics what FSX does with the "-c" option.
• Skip mmap-related tests when vfs.fusefs.data_cache_mode == 0

Sponsored by: The FreeBSD Foundation