Menu: convert system functional tests to PHPUnit

This should be done as a part of the System modules Menu tests conversion. There was no issue for that yet, so lets use this one.

This might be green....

Added '(active tab)' to the tabs test to make it pass, which adds nice coverage for accessibility which I think is a nice bonus.

+++ b/core/modules/system/tests/src/Functional/Menu/MenuRouterTest.php
index 0000000000..1ec1efee38
--- /dev/null

--- /dev/null
+++ b/core/modules/system/tests/src/Functional/Menu/MenuTestBase.php

+++ b/core/modules/system/tests/src/Functional/Menu/MenuTestBase.php
+++ b/core/modules/system/tests/src/Functional/Menu/MenuTestBase.php
@@ -0,0 +1,11 @@

@@ -0,0 +1,11 @@
+<?php
+
+namespace Drupal\Tests\system\Functional\Menu;
+
+use Drupal\Tests\BrowserTestBase;
+
+abstract class MenuTestBase extends BrowserTestBase {
+
+  use AssertBreadcrumbTrait;
+
+}

Maybe this is a little bit of a provocative question: If this is all MenuTestBase is doing, should we really have such a base class in the first place?

I think MenuTestBase is the only base class that just only has the trait. How about not converting this Base class during this conversion and directly use the BTB and use that trait in the child classes?

Yeah this is exactly what I've been thinking as well.

It looks like #11 did not include the fixes done in #7. Including them in this patch.

I had to manually check for these, hopefully I didn't miss anything.

Nice and minimal, everything left in /core/modules/system/src/Tests/Menu is deprecated, all tests have been moved out.

UNrelated fails

Committed/pushed to 8.5.x and cherry-picked to 8.4.x. Thanks!