Convert EntityViewsDataTest from a unit test to a kernel test

As this is going to be a lot of work, would be good to get a green light from a subsystem maintainer before starting work.

I'm making good progress on this.

- much less mocking to do, so much less code!
- the unit test actually uses entity type definitions from the entity_test module, which I find a bit weird... it's actually pretty simple to put the definitions in the test
- found a bug in the test!

    $this->assertEquals([
      'left_field' => 'id',
      'field' => 'entity_id',
      'extra' => [[
          'field' => 'deleted',
          'value' => 0,
          'numeric' => TRUE,
        ],
      ],
      // This is wrong! On a live site with the same entity type and fields enabeld, it joins to entity_test_mul_property_data!
    ], $data['entity_test_mul__string']['table']['join']['entity_test_mul']);

Done!

Thanks for your help with making the tests pass!

> Why add this todo? Can we not just do it? It looks simple enough
> Can we change this to an assertTrue().
> Can we change this to an assertArrayNotHasKey().

Because this is a test conversion, I think that the new test class needs to be as similar as possible to the unit test class we're removing, to help reviewers understand the changes and see clearly that the new test is still covering all the same things.

Changing the type of assertions will add a lot of noise to the diff and possibly make git diff stop seeing it as as a file rename, which will then make reviewing even harder.

I agree with the suggested changes to the assertions, but I think they should be done in a follow-up.

Removing the tag, as that was addressed in #5.