Port other-buffer

[simon-frankau]

Fixed #1476.

[simon-frankau]

I'm not quite sure how much testing to throw in. Fully testing other-buffer's functionality would require writing a fair chunk of tests (exercising the predicates, visibility, frame-local buffer lists, etc.), and my elisp knowledge is pretty weak. If you think it's worth it, I can add more, but I don't want to create a huge pile of tests for a single function if that looks silly.

[shaleh]

Are there tests in the expensive suite? The more tests we have the more flexibility we have to experiment with editor internals.

[shaleh]

It is ok to add simple ones and open an Issue for "feature X needs more testing".

[simon-frankau]

I couldn't see any further tests of other-buffer. I've added a test for the predicate, but the part with visibility requires much more understanding of how to test frames/windows than I currently have.

[simon-frankau]

I think I've made all the changes I intended to do here, and would appreciate someone taking another pass. (I'm also happy to make further changes as required.)

I'm also new to github review workflows, so now I'll try pressing the "re-request review" button and see if it does what I expect. :)

[agraven]

Some tests are failing, it looks like buffers aren't being named correctly for some reason. An example:

(ert-test-failed
  ((should (string= (buffer-name other) "1"))
      :form (string= "test-move-overlay-in-buffer-add-to-buffer" "1")