Incorrectly accessing posts fetched by WP_Query in query PHPUnit tests

[david.binda]

Similarly to #53280, the usage of WP_Query::get_posts() method after the WP_Query object was initialised with arguments passed to it is incorrect in tests/phpunit/tests/query/invalidQueries.php and tests/phpunit/tests/query/taxQuery.php PHPUnit tests, as it triggers just one more SQL query.

For instance: https://core.trac.wordpress.org/browser/trunk/tests/phpunit/tests/query/invalidQueries.php?rev=51568#L94 or https://core.trac.wordpress.org/browser/trunk/tests/phpunit/tests/query/taxQuery.php?rev=52389#L1027

Instead, the posts should be read via WP_Query::posts property.

[SergeyBiryukov]

In 52577:

Tests: Avoid duplicate queries in some WP_Query tests.

When passing args to the WP_Query::__construct() method, it internally executes the WP_Query::get_posts() method and stores the result in the WP_Query::posts property.

When calling WP_Query::get_posts() again, the same SQL query gets executed, and the result is again stored in the WP_Query::posts property.

Thus, the correct usage is to read the posts already stored in the WP_Query::posts property.

Follow-up to [1062/tests], [1065/tests], [1066/tests], [1070/tests], [29805], [31286], [49900], [51144].

Props david.binda.
Fixes #54822.