Fix assertFocus. This surfaces broken tests using assertFocusPath.

This is the PR I promised in #369. It fails a couple of tests that need fixing.

`assertFocus` was just asserting the current window has *a* name instead of
comparing it to the expected `name`.

Now that assertFocus really asserts, tests using assertFocusPath break:

 - test_slice_focus
 - test_zoomy_one

Ironically assertFocusPath makes the impression it asserts very thoroughly, by 
assertFocus twice in forward direction and twice backwards.