Split a couple of methods out of ManageFieldsFunctionalTest [#3612411]

Problem/Motivation

This is still one of the slowest functional tests, there are a couple of methods that are easy enough to split out.

Some of the test methods in this test don't do a form submission, so it might be a candidate to factor out parts into a kernel test once some of the other lower hanging fruit is converted.

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Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet

Issue fork drupal-3612411

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3612411-split-a-couple changes, plain diff MR !16384
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