Reason for adding DirectMountStrict is making the DirectMount
functionality testable, though it may have value for the user
in some cases.

Add defaultRawFileSystem & loopback tests for DirectMount
and DirectMountStrict.

The tests fail right now due to bugs in DirectMount that will be fixed
shortly:

	go-fuse/fuse$ sudo /usr/local/go/bin/go test -run TestDirectMount
	[...]
	2022/12/28 20:19:21 mountDirect: calling syscall.Mount("", "/tmp/TestDirectMount3242971772", "fuse./tmp/go-build1215740", 0x0, "fd=7,rootmode=40000,user_id=0,group_id=0")
	2022/12/28 20:19:21 mount: failed to do direct mount: invalid argument
	[...]

Change-Id: Ibfa2fa141cb43e1f8c7319233c454a3e85fa435e