test: use flock when calling mkfs.btrfs
    
    As stated in https://github.com/systemd/systemd/issues/21819#issuecomment-1064377645
    `mkfs.btrfs` doesn't hold the lock for the whole duration of
    `mkfs.btrfs`, thus causing unexpected races & test fails. Let's
    wrap the `mkfs.btrfs` calls in an flock wrapper to mitigate this.
    
    Hopefully fixes: #21819