WP_UnitTestCase::go_to() wipes out custom query vars

[johnbillion]

The go_to() method in the test suite re-instantiates the $wp global with a new WP object. This means that any custom query vars that have been added via $wp->add_query_var() get wiped out, making them untestable.

An affected function is add_rewrite_endpoint() which internally calls $wp->add_query_var().

This test demonstrates the problem:

$test_post_id = self::factory()->post->create();

add_rewrite_endpoint( 'foo', EP_PERMALINK );
flush_rewrite_rules();

$this->go_to( get_permalink( $test_post_id ) . 'foo/bar/' );

$this->assertTrue( is_single( $test_post_id ) ); // passes
$this->assertEquals( 'bar', get_query_var( 'foo' ) ); // fails

[johnbillion]

In 35258:

Preserve the public and private query var properties on the global WP instance when using WP_UnitTestCase::go_to(). These properties apply to the application state, not the current request.

See #34346