De-duplicate logic in ManageFieldsFunctionalTest

Problem/Motivation

ManageFieldsFunctionalTest is very old and one of the very longest running functional tests at more than three minutes.

We don't need ::testDeleteTaxonomyField() when we have the dedicated FieldUIDeleteTest that does a lot more. Ideally all of this would use entity_test instead of taxonomy and node modules but that would be a much bigger refactoring.

A couple of test methods can also be combined into one as well which will reduce runtime while still testing similar functionality.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet