wp-phpunit: go_to() does not unset current_screen

[mauteri]

current_screen should be added to this array for unsetting: ​https://github.com/wp-phpunit/wp-phpunit/blob/master/includes/abstract-testcase.php#L710

As of now, if you set_current_screen (which is automatically done in WP_Ajax_UnitTestCase to set_current_screen( 'ajax' )) is_admin() will be true when you tell your test, for example, $this->go_to( '/' ); A test like this will fail:

$this->go_to( '/' ); // go to homepage
$this->assertFalse( is_admin() ); // fails because is_admin() is true

[SergeyBiryukov]

In 45486:

Build/Test Tools: Add $current_screen to the list of globals to unset in WP_UnitTestCase_Base::go_to().

Props mauteri.
Fixes #47439.