tests/file_systems: Move some tests to a common "shared" directory.

Rather than having individual directories for the tests. No
functional change intended.

Change-Id: Ie453eab2d7347cef4ea68a1e7793d01817e7da39
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7706
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>