Improve organiation of tax_query and meta_query unit tests.
meta_query tests have been moved to tests/phpunit/tests/query/metaQuery.php and tax_query tests to tests/phpunit/tests/query/taxQuery.php. This is an improvement because (a) it better corresponds to the way that other WP_Query parameter tests are organized, (b) splitting meta and tax tests into separate classes simplifies the required @group annotations, and (c) the tests have nothing to do with posts per se, and so do not belong in the post subdirectory.
The tests previously found at tests/phpunit/tests/query/taxQuery.php have been moved to isTerm.php in the same directory. These tests are related to the is_* functions that have to do with taxonomy terms, like is_category().
See #26999.