selftests: add tests for mount notification

Provide coverage for all mnt_notify_add() instances.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250307204046.322691-1-mszeredi@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>