Fix resource leak in tests.

Change-Id: I14faea81a7c14d39b9bb6e07d1b1dce4dcf3183d

fs/gitilesfs_test.go

diff --git a/fs/gitilesfs_test.go b/fs/gitilesfs_test.go
index 35f5865..bd53d25 100644
--- a/fs/gitilesfs_test.go
+++ b/fs/gitilesfs_test.go
@@ -428,6 +428,7 @@
 	if err != nil {
 		t.Fatalf("newManifestTestFixture: %v", err)
 	}
+	defer fix.cleanup()
 
 	fs := fix.root.(*manifestFSRoot)
 	ch := fs.Inode()