btrfs: tests: allocate dummy fs_info and root in test_find_delalloc()

[ Upstream commit b2136cc2 ]

Allocate fs_info and root to have a valid fs_info pointer in case it's
dereferenced by a helper outside of tests, like find_lock_delalloc_range().

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>