Remove \Drupal\FunctionalTests\Update\UpdatePathTestBase::$installProfile, use the inherited $profile property instead

Problem/Motivation

Discovered in #2352949-74: Deprecate using Classy as the default theme for the 'testing' profile, quoting verbatim:

Apparently UpdatePathTestBase does not just use the DB dump, it does still install Drupal … to some extent. And because UpdatePathTestBase sets protected $installProfile = 'standard'; and simply ignores the inherited protected $profile = 'testing';, this results in the FunctionalTestSetupTrait-added code also executing for update path tests, which then results in the default theme being overridden 🤦‍♂️

I think we should probably get rid of \Drupal\FunctionalTests\Update\UpdatePathTestBase::$installProfile, but that's definitely out-of-scope here.

This is that issue.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet