Tests: Stop using deprecated setMethods().

[GaryJones]

As per sebastianbergmann/phpunit#3687 setMethods() is deprecated and will be removed in PHPUnit 10.

The option is to use onlyMethods() instead, but this needs checking to see if PHPUnit-Polyfills handles this for PHPUnit 7, which we still need to test PHP 7.1.

We only have a single instance of setMethods(), so we may be able to fashion a workaround if it's not polyfilled.

[GaryJones]

As shown at https://github.com/Parsely/wp-parsely/runs/3754851373?check_suite_focus=true the onlyMethod() method is not polyfilled.

I've opened Yoast/PHPUnit-Polyfills#58 to see if it can be polyfilled.

Untested, but I think a workaround would be like:

$mock_builder = $this->getMockBuilder( Integration::class );
if ( method_exists( $mock_builder, 'onlyMethods' ) ) {
	$mock_integration = $mock_builder->onlyMethods( array( 'integrate' ) )->getMock();
} else {
	$mock_integration = $mock_builder->setMethods( array( 'integrate' ) )->getMock();
}