ContentTranslationStandardFieldsTest should use API to verify fields

Neat find!

That test was added in #2290849: Regression: Fields missing from translation overview since "Clarify handling of field translatability" → .

It seems like Drupal\Tests\standard\Functional\StandardTest::testStandard() could absorb this test pretty easily.

That issue is a mixture of fixes.

- fixing missing translatable setting in some fields installed by standard profile
- comment manager's addDefaultField() missing translatability.

Checking the standard profile via a full installation is silly -- we're going via a huge expensive installation just to check values in one YAML got loaded into config. We already know the config system works. If we want to check that fields installed by the standard profile have their translatability set up correctly, a unit test could just read the YAML files.

As for CommentManager, addDefaultField() was removed in #2357199: Consider CommentManagerInterface::addDefaultField() as deprecated and remove in favour of CommentTestTrait → with no replacement, so I don't think there's anything needed to test there any more.

> Checking the standard profile via a full installation is silly -- we're going via a huge expensive installation just to check values in one YAML got loaded into config.

I see that StandardTest is doing just that -- installing the profile & then checking settings in the UI admin forms.

In the interest of getting this issue fixed, let's fold it into that as @danflanagan8 suggests. Arguing about the sense of StandardTest can be another issue :)