[Tests] Dropped providing fallback Kernel if not defined (#345)

For more details see #345

Key changes:

* [Tests] Dropped providing fallback Test Kernel if not defined

* [Tests] Made IbexaTestKernel and IbexaKernelTestCase internal
  For both 1st and 3rd party, contracts from ibexa/test-core need to be used instead.

src/contracts/Test/IbexaKernelTestCase.php

@@ -8,22 +8,12 @@
 
 namespace Ibexa\Contracts\Core\Test;
 
-use LogicException;
 use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
 
 /**
- * @experimental
+ * @internal For core tests only. Use \Ibexa\Contracts\Test\Core\IbexaKernelTestCase from ibexa/test-core instead.
  */
 abstract class IbexaKernelTestCase extends KernelTestCase
 {
     use IbexaKernelTestTrait;
-
-    protected static function getKernelClass(): string
-    {
-        try {
-            return parent::getKernelClass();
-        } catch (LogicException $e) {
-            return IbexaTestKernel::class;
-        }
-    }
 }

src/contracts/Test/IbexaTestKernel.php

@@ -34,7 +34,7 @@
 use Symfony\Component\HttpKernel\Kernel;
 
 /**
- * @experimental
+ * @internal For core tests only. Use \Ibexa\Contracts\Test\Core\IbexaTestKernel from ibexa/test-core instead.
  *
  * Baseline test kernel that dependent packages can extend for their integration tests.
  *