* 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