Update Taxonomy tests to not use Bartik and Seven

Problem/Motivation

There are several tests that have references to Bartik and Seven:

      3 core/modules/taxonomy/tests/src/Functional/Rest/TermResourceTestBase.php
   6 core/modules/taxonomy/tests/src/Functional/ThemeTest.php

These tests should be updated to either use Olivero, Claro, or System module's test_theme so we can deprecate Bartik #3249109: Deprecate Bartik and Seven #3084814: Deprecate Seven theme as mentioned at #3278124: Convert various tests that use bartik/seven to olivero/claro.

Steps to reproduce

git grep -E '(bartik)|(seven)' -- 'core/modules/taxonomy/tests' | awk -F: '{print $1}' | sort | uniq -c should return no results when this work is complete.

Remaining tasks

Update the tests.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

I don't think we need release notes or a change record.