Do not override container with itself in DrupalUnitTestBase::containerBuild()

When we call DrupalUnitTestBase::setUp(), before $this->containerBuild(drupal_container()); we call parent::setUp();, that calls TestBase::prepareEnvironment(), that sets container into the test object - \Drupal::setContainer($this->container);. So DrupalUnitTestBase::containerBuild() makes some overheads.