Fix for #1381: Fail test with warning if the developer does not make the...

...ir test inherit from PHPUnit_Framework_TestCase or PHPUnit_Framework_TestSuite
For an early check in the `createTest()` method, testing for the `PHPUnit_Framework_Test` interface should be sufficient.
```php
if (!$theClass->implementsInterface('PHPUnit_Framework_Test')) {
```
More granular inheritance checks may happen in subsequently called code (which might be custom implementations).