Replace phive with composer

This is sadly necessary as phive doesn't take the current PHP-Version
into account. So it installs the latest and best regardless whether that
works with the current PHP-Version. Like PHPUnit10 and PHP7.4 :-/

.github/workflows/tests.yml

@@ -30,9 +30,9 @@ jobs:
       - name: install dependencies
         run: composer install
       - name: install tools
-        run: phive install --trust-gpg-keys 4AA394086372C20A phpunit
+        run: composer require --dev phpunit
       - name: Run Unit-Tests
-        run: ./tools/phpunit --testdox
+        run: ./vendor/bin/phpunit --testdox
   coverage:
     needs: test
     runs-on: ubuntu-latest