[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/Predis/ClientExceptionTest.php

@@ -21,7 +21,7 @@ class ClientExceptionTest extends PredisTestCase
     /**
      * @group disconnected
      */
-    public function testExceptionMessage()
+    public function testExceptionMessage(): void
     {
         $message = 'This is a client exception.';
 
@@ -34,7 +34,7 @@ class ClientExceptionTest extends PredisTestCase
     /**
      * @group disconnected
      */
-    public function testExceptionClass()
+    public function testExceptionClass(): void
     {
         $exception = new ClientException();