[Notifier] Move abstract test cases to namespace

This enables BC promise for the abstract test classes, like for other ones.

As we ignore current `/Tests` directory in `.gitattributes`, builds will fail if `symfony/notifier` is installed as dist.

Author: OskarStark

Tests/TransportFactoryTestCase.php renamed to Test/TransportFactoryTestCase.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Symfony\Component\Notifier\Tests;
+namespace Symfony\Component\Notifier\Test;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Notifier\Exception\IncompleteDsnException;

Tests/TransportTestCase.php renamed to Test/TransportTestCase.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Symfony\Component\Notifier\Tests;
+namespace Symfony\Component\Notifier\Test;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Notifier\Exception\UnsupportedMessageTypeException;