test: Improve colon test comment

I use these patterns when early hacking, and it's good to have a test to
catch them all before I merge.

engine/resources/resources_test.go

@@ -1688,7 +1688,7 @@ func TestResources2(t *testing.T) {
 			fileAbsent(d2f1),
 			fileAbsent(d2f2),
 			fileAbsent(d2f3),
-			fileExists(p2, false), // ensure it's a file XXX !!!
+			fileExists(p2, false), // ensure it's a file
 			fileExists(p3, true),  // ensure it's a dir
 			fileExists(p4, false),
 			resCheckApply(r1, true), // it's already good