Merge ManageFieldsUiTest into other field_ui tests and remove it along with ManageFieldsUiTestBase

Problem/Motivation

Postponed on 📌 Ensure ManageFieldsLifecycleTest::testCrudFields() test coverage is covered by ManageFieldsTest then delete it Needs work and 📌 Ensure ManageFieldsMultipleTypes test coverage is covered by ManageFieldsTest then delete it Needs work .

See 📌 Remove duplicate test coverage from ManageFieldsUiTest Needs review for original discussion.

Most of the test coverage provided by this class duplicates more recent test coverage added elsewhere, if it's not duplicated, we could probably move it to the newer classes and then still remove ManageFieldsUiTest to have more logical groupings and less cross-subsystem test dependencies.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet