Replace deprecated method of PHPUnit

[vitormattos]

The PHPUnit method setMethod was replaced by onlyMethod. setMethod was used to create a double of methods that exist and don't exist. Replacing all occurrences of this method, I found a test case creating mock of a method that does not exist.

The method Invitation::parseInvitation was moved to Notification::parseInvitation.

More information: sebastianbergmann/phpunit#3687

Steps to reproduce

1. replace all occurrences of setMethod by onlyMethod
2. Run tests
3. You will receive:

1) OCA\Talk\Tests\php\Activity\Provider\InvitationTest::testParse with data set #0 ('en', true, array(23, 'test1'), array(array('actor-data'), array('call-data')))
PHPUnit\Framework\MockObject\CannotUseOnlyMethodsException: Trying to configure method "parseInvitation" with onlyMethods(), but it does not exist in class "OCA\Talk\Activity\Provider\Invitation". Use addMethods() for methods that do not exist in the class

/var/www/html/apps/spreed/tests/php/Activity/Provider/InvitationTest.php:85
/var/www/html/apps/spreed/tests/php/Activity/Provider/InvitationTest.php:142

2) OCA\Talk\Tests\php\Activity\Provider\InvitationTest::testParse with data set #1 ('de', false, array(42, 'test2'), array(array('actor-data'), array('call-unknown')))
PHPUnit\Framework\MockObject\CannotUseOnlyMethodsException: Trying to configure method "parseInvitation" with onlyMethods(), but it does not exist in class "OCA\Talk\Activity\Provider\Invitation". Use addMethods() for methods that do not exist in the class

/var/www/html/apps/spreed/tests/php/Activity/Provider/InvitationTest.php:85
/var/www/html/apps/spreed/tests/php/Activity/Provider/InvitationTest.php:142

Server configuration

PHP version: 7.4

Nextcloud Version: 23

PHPUnit version: 9.5.9