vfstest: split idmapped mount tests into separate suite

Now that we finished restructuring split the core idmapped mounts test
suite into a separate source file.

Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>
4 files changed