Remove TestSetupTrait::$originalProfile and test infrastructure which uses it

I also noticed the pointlessness of $originalProfile while working on #2352949: Deprecate using Classy as the default theme for the 'testing' profile!

+1 to removing it.

I'm inclined to agree with

This just makes it less magical and more obvious.

… but this does seem like it could break some really obscure tests. So tagging for release manager review.

Actually, that was probably premature. A subsystem maintainer is more likely to be able to thoroughly and efficiently assess the impact of this change.

Also, I think this needs a change record.

Sorry, @Mile23!

As a subsystem maintainer +1

I think this is mostly right in that people think they have two profiles when they're writing the test. The thing is: They really don't. This just makes it less magical and more obvious.

Also I disagree that we need a CR here as there is nothing for someone to change - $originalProfile is set by the system and not the test developer. If this does cause a test failure then you're probably not testing what you think you're testing. Also note core almost certainly has the most complex set of install profile tests anywhere and this change does not cause us to have to change any tests.

See, that's exactly why I wanted a subsystem maintainer to review this — because I don't know enough about this to assess whether it's truly RTBC-worthy or not :) Thanks, @alexpott!

Given #11 which addresses my reasons for in-RTBC'ing in #10, I'm now comfortable RTBC'ing this like I did in #9 :)

Committed 4e3cc12 and pushed to 8.8.x. Thanks!

+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -774,15 +774,6 @@ protected function moduleData($module) {
-      // If a module is within a profile directory but specifies another
-      // profile for testing, it needs to be found in the parent profile.
-      $parent_profile = Settings::get('test_parent_profile');
-      if ($parent_profile && !isset($profiles[$parent_profile])) {
-        // In case both profile directories contain the same extension, the
-        // actual profile always has precedence.
-        $profiles = [$parent_profile => $all_profiles[$parent_profile]] + $profiles;
-      }
-

<3 This makes me happy. One less way of doing super strange low-level things.

diff --git a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
index 71fdb4f401..60b08cc8b5 100644
--- a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
@@ -4,7 +4,6 @@
 
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
-use Drupal\Core\Site\Settings;
 use Drupal\Core\State\StateInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 

Removed unused use on commit.