Remove URL generator priming in \Drupal\Tests\BrowserTestBase::installDrupal()

Problem/Motivation

In #2189411: Remove an unnecessary container rebuild from FunctionalTestSetupTrait we removed a legacy call to drupal_flush_all_caches() but this had the unintended impact of causing \Drupal\Tests\node\Functional\Views\FrontPageTest to fail. This is because it uses \Drupal\views\Tests\AssertViewsCacheTagsTrait::assertViewsCacheTags() which is primarily designed to work in kernel tests. It does $request = new Request(); whereas it should be $request = Request::createFromGlobals(); so that it uses the $_SERVER settings done in \Drupal\Core\Test\FunctionalTestSetupTrait::setupBaseUrl(). Because it pushes an incorrect request onto the request stack, if the RequestContext is not yet initialised, the url generator with not be able to produce absolute urls.

Proposed resolution

Remove Url::fromRoute('<front>')->setAbsolute()->toString();.

Contrib / custom tests might need to be fixed but if so they are pushing a request to the request stack with incorrect info.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

@todo