Missing cache invalidation / clearing when updating site_template and variant entities

This is the latest patch from #2914579: Improve the tests for the D8 branch with the cache-clear workarounds removed from the test-cases.

Created a patch to solve the cachability meta-data problem.

Note:

• One of the test-cases fails is because of #2902488: Cannot use object of type Drupal\Core\StringTranslation\TranslatableMarkup as array and therefore the patch attached is a version with that fix included. I'll reroll once that fix is commited.
• I added back parts of the workarounds to be able to only test & fix problems in regards to cachability meta-data.

Oh .. forgot to attach the files ... here they are.

I did the diff wrong for the test-only patch. Here is a correct one.

Rerolled the patch

Now on to the next part:

• Fixed my coding standards violations (see interdiff)
• Removed the remaining workarounds to show what is still missing (see other interdiff)

This fixes the test fails.

It adds cache clears after enabling site_template and display variants to ensure that page which were cached before that happened and which are after that being served are properly cleared. This is necessary because they are not automatically invalidated as they were not dependent on panels_everywhere related config-entites before.

I also just thought of a another edge-cache:
Modifying a page variant to cover additional pages that where not served by panels_everywhere before.
I will try to write a test that proves this assumption.

Here is a patch with the additional test-cache that proves my assumption.

Will Fix that now.

Here is a fix for #15.

This needs to be rerolled after #2914579: Improve the tests for the D8 branch is in.

Just realized that the last patch introduces coding standards violations. Fixing that with with this patch.

@DamienMcKenna: If you want, I can fix the remaining violations during the upcoming reroll.

Back to postponed until #2914579: Improve the tests for the D8 branch is in.

Rerolled the patch.

I cleaned up some comments and fixed the new coding style violation.

This last patch fixes the remaining coding standards violations in all files affected by the patch.

The previous reroll lost the caching-metadata being passed to pe-display-variant, as such the variant has not meta-data to apply.

This patch fixes that & also cleans up the process a bit.

Reverted some changes made to the failing test in previous patches.

After doing some manual testing I actually realized that the concept for this patch is false.

After all some of the tested functionality does work without the fixes from the recent patch.

I'll investigate this further.