btrfs: tests: Fix a memory leak in error handling path in 'run_test()'

If 'btrfs_alloc_path()' fails, we must free the resources already
allocated, as done in the other error handling paths in this function.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>