Tests: Use the factory method instead of the property.

This replaces all references to the `WP_UnitTestCase_Base::$factory` property with static function calls to the `WP_UnitTestCase_Base::factory()` method.

This is a consistency improvement for the test suite.

Follow up to [35225], [35242], [49603], [54087], [54088].

Props jrf.
See #55652.

git-svn-id: https://develop.svn.wordpress.org/trunk@54090 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/customize/nav-menus.php

@@ -417,7 +417,7 @@ class Test_WP_Customize_Nav_Menus extends WP_UnitTestCase {
 			)
 		);
 
-		$term_id = $this->factory->term->create(
+		$term_id = self::factory()->term->create(
 			array(
 				'taxonomy' => 'wptests_tax',
 				'name'     => 'foobar',