btrfs: test that we can not delete a subvolume with an active swap file

btrfs: test that we can not delete a subvolume with an active swap file

Verify that we can not delete a subvolume that has an active swap file,
and that after disabling the swap file, we can delete it.

This tests a fix done by kernel commit 60021bd754c6ca ("btrfs: prevent
subvol with swapfile from being deleted"), which landed in kernel 5.18.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reivewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>