units: Don't include the test directory in the output tags file

Not having the test directory name in the expected output makes it easier to move the test case, e.g. from a `.i` or `.b` directory (broken tests) to a `.d` one (normal test) e.g. when fixing a parser bug.

---

When modifying the CSS parser I noticed it was a bit frustrating to mark a test case as fixed, because as the expected output contains the test's directory name, it's not as simple as renaming the directory from `.b` or `.i` to `.d`, but it also requires editing the *expected.tags* to change the directory name there.

So, what I propose here is to strip the directory name from the expected output so renaming a test directory doesn't break the test.

I implemented this as a filter as it was the simplest way.  I first wanted to run the ctags command in the subdirectory, but this becomes a mess because of relative paths, and would at least require large changes.