[tests] Start improving test suite.

- Make use of more typehints for function parameters
- Make use of typehints for function return values
- Use @var where needed to give proper hints to IDEs and avoid warnings
- Replace MockObject::setMethods() with addMethods() and onlyMethods()
- Rewording of some phpdocs

tests/PHPUnit/ArrayHasSameValuesConstraint.php

@@ -10,7 +10,7 @@
  */
 
 /**
- * Constraint that accepts arrays with the same elements but different order.
+ * PHPUnit constraint matching arrays with same elemnts even in different order.
  */
 class ArrayHasSameValuesConstraint extends \PHPUnit\Framework\Constraint\Constraint
 {
@@ -19,7 +19,7 @@ class ArrayHasSameValuesConstraint extends \PHPUnit\Framework\Constraint\Constra
     /**
      * @param array $array
      */
-    public function __construct($array)
+    public function __construct(array $array)
     {
         $this->array = $array;
     }