Remove superseded unit tests for balanceTags()

[coffee2code]

Now that #1597 and #20401 have (long since) been committed, the unit tests relating to balanceTags() can be updated to:

• Remove tests that existed solely to test the older, more limited tag handling of the function. Tests already exist that supersede those, testing the currently more comprehensive tag handling. Inline comments confirm they can now be removed.
• Remove references to those now-closed tickets

FYI, I was the originator of all of the tests in tests/formatting/balanceTags.php so these changes are in keeping with what was originally planned when I wrote them.

Additionally, while I was in there I fixed the dataProvider function nestable_tags() which wasn't providing the data properly. No harm since it wasn't being used anyhow, but I went ahead and adapted one of the existing tests to make use of it.

Patch attached.

[nacin]

Looks good. We keep @ticket annotations even after the tickets are closed. Once the ticket is closed, the test runner knows not to skip them.

[nacin]

In 27040:

Remove duplicated and superceded balance tags unit tests.

props coffee2code.
fixes #26222.