Rethink "upgrade tests"

[dafeder]

In our CI, we have one set of jobs that are supposed to test upgrades. However, the PHPUnit tests are no longer valid upgrade tests; we have switched to using BrowserTestBase which does a clean install for every test, so there is no way a test would fail on upgrade but not on install. Previously, we used Drupal Test Traits to test against an existing instance, so we could get the code + db in the correct state to simulate an upgrade.

Let's get something in place to restore this, as we've had a few recent bugs or regressions introduced that proper upgrade tests would have caught. We don't really want to re-add DrupalTestTraits as a dependency but could possibly use the core BuiltTest framework, or test Drush and API output via some other test framework outside of Drupal PHPUnit tests (something bash-based perhaps, or kernel-based tests that use a pre-build sqlite fixutre... there could be many approaches).