generic: overlay group ownership changing test

generic: overlay group ownership changing test

This's copied from generic/698, extend it to test overlayfs on top of
idmapped mounts specifically.

When group ownership is changed a caller whose fsuid owns the inode can
change the group of the inode to any group they are a member of. When
searching through the caller's groups we failed to use the gid mapped
according to the idmapped mount otherwise we fail to change ownership.
Add a test for this.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>