Translate routes should properly inherit admin path use from edit route

Better title.

Adding test.

Well, I manually tested the fix while working on it and it manually worked :) So I'm wondering how this is not good :/ I'll see to add some more asserts for the edit page too to make sure alignment is happening.

I thought that routes need to be rebuilt when the theme settings are set, but that seems to be already happening:

function node_form_system_themes_admin_form_submit($form, &$form_state) {
  \Drupal::config('node.settings')
    ->set('use_admin_theme', $form_state['values']['use_admin_theme'])
    ->save();
  \Drupal::service('router.builder')->setRebuildNeeded();
}

So it is a possibility that the JS snippet I use to test for the applied theme does not appear in the test system for some reason. Hum. Anyway, adding a condition for the edit page as well to see if that at least follows the setting or not in the test.

So this proves my rebuilding theory. While the submit function I quoted above does have rebuild, our test uses the config API directly and therefore does not test how Drupal would normally react when this change is flipped. We should test through the UI and set the mode there so the route rebuild is going to happen.

So the final problem was the test did not allow admins to see the admin theme. Doh. That took a bit too much time... Removed testing the edit page because that is already tested in ThemeTest.php in system module. I looked into extending that to test for translate, but there are several things that need to be set up, translatability, etc. for the tab to show up, so better not merged this into system's ThemeTest.php.

I think this is ready now :)

Yeah the assumption that that option makes it an admin route is already encoded in the other parts of this route provider, so while we can make it look a tiny bit nicer here, we are directly fiddling with routes here.

+++ b/core/modules/content_translation/src/Routing/ContentTranslationRouteSubscriber.php
@@ -63,6 +69,7 @@ protected function alterRoutes(RouteCollection $collection) {
+          '_admin_route' => $is_admin,

This. In multiple copies.

13: 2276387-tranlate-path-theme-13.patch queued for re-testing.

#1987882: Convert content_translation routes to a new style controller just attempted to hardcode translate routes as admin routes. Would be good to let this land sooner than later. :)

Updated remaining tasks.

Woot, superb, thanks.