Simplify defining test namespaces

[mglaman]

See https://git.drupalcode.org/project/drupal/-/commit/d39f152b8ba6141d63d194a1be84218046b6a2e9

[mglaman]

This is already done, only Drupal core has a verbose namespace registration

            // Extensions can have a \Drupal\Tests\extension namespace for test cases, traits, and other classes such
            // as those that extend \Drupal\TestSite\TestSetupInterface.
            // @see drupal_phpunit_get_extension_namespaces()
            $module_test_dir = $module_dir . '/tests/src';
            if (is_dir($module_test_dir)) {
                $this->namespaces["Drupal\\Tests\\$module_name"] = $module_test_dir;
            }