RouteBuilderTest doesn't cover integration with the rest of core, and RouteBuilder has no kernel test coverage

Problem/Motivation

Over at #2976861: add an entity Links handler, to provide menu links/tasks/actions for entity types I've got a test failure to do with circularity in the route builder / menu links system. However, the only kernel test that failed is \Drupal\Tests\views\Kernel\TestViewsTest.

I had a look at it, and I don't think it's Views doing anything weird. So we should be seeing a test failure in a routing system test too. But we're not.

The only test I could find for the RouteBuilder is a unit test, RouteBuilderTest. Since it mocks all the class's dependencies, it's not going to pick up anything like circularity with the links system, for example.

So it looks like we're missing test coverage here.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet