Improve visual diff tests

Now there are separate test cases for:
 - A single target with no diff
 - A single target with some diff
 - A single target with some diff, which is case only
 - Multiple targets, some with diffs
 - Multiple targets, some with diffs, but only in case

Furthermore, the used sample diff strings actually look like
real diff strings, for better readibility.

Also, removed some useless lines from the code.

(Thx @aron for all the suggestions.)