Improve unit tests for WP_Tax_Query.

* Exhaustive tests for publicly available functionality of WP_Tax_Query.
* For tests that are related to the tax_query argument as used in WP_Query, move to tests/post/query.php.
* Add some tax_query tests to cover single vs multiple queries using AND and OR; various values for 'field'; various values for 'operator'.
* Improve test names.
* Correct @group annotations.
* Improve performance of some WP_Query-related tests by declaring 'update_post_meta/term_cache' false.

Fixes #29718

git-svn-id: https://develop.svn.wordpress.org/trunk@29805 602fd350-edb4-49c9-b593-d223f7449a82