[PHP] Add PreferThisOrSelfMethodCallRector

[TomasVotruba]

Closes #863

# rector.yml
services:
    Rector\Php\Rector\MethodCall\PreferThisOrSelfMethodCallRector:
        PHPUnit\Framework\TestCase: 'this' # or 'self'

 final class MyTestCase extends PHPUnit\Framework\TestCase
 {
     public function test()
     {
-        self::assertNull();
+        $this->assertNull();
     }
 }