Update tests whose assertions are implicitly relying on Classy markup to not rely on it anymore

Problem/Motivation

1. #2352949: Deprecate using Classy as the default theme for the 'testing' profile set a new direction for how to avoid having our tests be dependent on themes that may disappear. It also introduced a BC layer. For now, Drupal core uses that BC layer, and because of that deprecation errors are triggered.

   They were suppressed thanks to \Drupal\Tests\Listeners\DeprecationListenerTrait::getSkippedDeprecations().

2. #3082655: Specify the $defaultTheme property in all functional tests removed those suppressions of deprecation errors and instead updating all relevant tests. It did so WITHOUT changing assertions.
3. But there's one more thing we can do to minimize the number of tests using classy: modify assertions to not depend on Classy markup. This is possible for lots of tests that aren't explicitly testing Classy.

Proposed resolution

Execute point 3 above.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet