[CodeQuality] Add ChangeMockObjectReturnUnionToIntersectionRector

[TomasVotruba]

Adds code quality rule that converts a @return docblock union with MockObject into an intersection type.

 /**
- * @return Event|\PHPUnit\Framework\MockObject\MockObject
+ * @return Event&\PHPUnit\Framework\MockObject\MockObject
  */
 private function getEvent(): \PHPUnit\Framework\MockObject\MockObject
 {
     return $this->createMock(Event::class);
 }

Mirrors the existing AddIntersectionVarToMockObjectPropertyRector (for @var), applied to method @return tags in test classes.

Registered in the phpunit-code-quality set.