vfstest: split out btrfs idmapped mounts test

Split the btrfs specific idmapped mounts test 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