vfstest: split out btrfs idmapped mounts test

Split the btrfs specific idmapped mounts test into a separate source
file.

Cc: Dave Chinner <david@fromorbit.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Eryu Guan <guaneryu@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Zorro Lang <zlang@redhat.com>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: fstests <fstests@vger.kernel.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>