Write functionality tests

Added a few functional tests, plus some minor phpcs and optimization fixes.

For discussion:
- Do we need to invalidate the page cache after saving the settings form?
- The install test is redundant as it is included in multiple other tests, should we remove it?

Thanks, @alayham! Could you provide your patch as an MR? I'd appreciate that!

@Grevil Why?
I have not done much development in Drupal for a while, is there a rule against using patch files that I missed?
I have downloaded the module and started writing tests without creating a fork, when done I thought the patch file is clear and readable and I did not know of any rule or decision against using patch files.

It is generally way easier to review, since you have the beautiful Gitlab UI at your hands, which gives you highlighted diffs, the ability to comment changes, a WEB IDE and further features like automatically rebasing your code to the current dev release, which helps a lot, if an issue is open for a while.

I'll quickly create one for you.

Done. Also since DrupalCI is deprecated and will be replaced with Gitlab CI → , no tests will run on a provided patch.

Back to "Needs review".

@alayham thanks! I reviewed this basicylly. @Grevil did you already take a look at the tests?

Needs a follow-up for the cache thing, see comment.

@Anybody no I didn't have the time yet!

The followup issue link is in comment 13, here it is again
🐛 Should we Invalidate the page cache after saving settings? Active

Automatically closed - issue fixed for 2 weeks with no activity.