Ensure ManageFieldsMultipleTypes test coverage is covered by ManageFieldsTest then delete it

Problem/Motivation

ManageFieldsUiTest was added in 2009(!) it has not dramatically changed.

Since it was added, we added FieldUiDeleteTest in 2015 in #2575605: Field delete form does not display configuration dependencies that will be updated/deleted → which has comprehensive coverage for field deletion, and also doesn't depend on taxonomy or node modules.

Also testCrudFields() is mostly duplicated by the much more modern ManageFieldsTest, which uses entity_test module instead of article/basic page/tags fields etc.

We should check for coverage in ManageFieldsMultipleTypes that's not in ManageFieldsTests or other field_ui tests, add anything over that makes sense, then delete it to reduce the duplication.

If we want coverage of multiple entity types just in case, we can just ensure two different ones are tested, not 3-5 as currently in field_ui test. And/or add some basic testing to node/taxonomy etc.modules themselves.

Steps to reproduce

Proposed resolution

Remove the two test methods from ManageFieldsUiTest and leave the more comprehensive coverage in FieldUiDeleteTest.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet