Use UnitTestCase as parent class for Unit tests

Drupal\Tests\examples\Unit\RoutePathTest and Drupal\Tests\examples\Unit\YamlValidationTest use PHPUnit\Framework\TestCase as parent class.

They should instead have the Drupal\Tests\UnitTestCase class as parent class, since that class ensures that an instantiated container in the global state of \Drupal from a previous test does not leak into the Unit test.