Make Gitlab D11 PHPUnit job run successfully

Well... when I wrote the initial spec, I assumed that "current version" would stay on D10, and D11 would stay "next major version", which would not cause the test pipeline to go red.

That wasn't true. D11 has been promoted to "current version" and tests are red now.

So I had a good look at the tests, removed the D11-incompatible dependencies, commented out relevant tests, and did a writeup in custom_elements_test_paragraphs/README.md about why this is no problem in principle... IF we get more complete test coverage elsewhere.

I'm re-titling this issue and keeping it open for that purpose / thinking about that.

Meanwhile, I'll commit the MR myself, whenever I need a pipeline to go green (e.g. to prove out that another issue is in fact OK). It contains only busywork on test(module)s.

This retitling implies a change of scope too, from "make Gitlab pipeline work" (though that may still be needed for 8.x-2.x) a to a broader "fuller test coverage" issue. At least to make sure that url fields and entityreference formatters have good solid test coverage, as the issue description now says.

The I'll need to go through existing test issues (or "only still open because the tests were not finished / I should recheck if all requirements are met" issues), to see if they are effectively child issues of this one now.

"Needs review" state is temporary, until the MR is committed, then it goes back to Active.

Also, IMHO we should make tests run on D10 too. And maybe on different PHP versions.

I don't exactly know how it works / I'm not going to look right now, but it should be easy enough; japerry did some stuff in my samlauth module → that has it running loads more versions as its standard pipeline.

A new issue 🐛 Referenced entity in wrong language Active came in for 8.x-2.x, which proved that indeed, its Github pipeline is failing too.

I've used the draft MR there to make the pipeline succeed again, and will push it to 8.x-2.x directly.

Now that I've looked into .gitlab-ci.yml... I'll use this MR to test a bunch of other things on 3.x additionally. D10 is the most important one, but since there are a lot of other options we can easily enable: why not...

  OPT_IN_TEST_PREVIOUS_MAJOR:   ### This is D10, we definitely need this.
  OPT_IN_TEST_PREVIOUS_MINOR:  
  OPT_IN_TEST_NEXT_MINOR: 
  OPT_IN_TEST_NEXT_MAJOR:          
  OPT_IN_TEST_MAX_PHP: 

Currently running (Jobs are enabled/disabled by Drupal gitlab as needed)

OPT_IN_TEST_CURRENT:         11  
OPT_IN_TEST_MAX_PHP:         Looks to me like PHP 8.3.12 == same as CURRENT. Shrug.
OPT_IN_TEST_NEXT_MINOR:      11.x-dev
OPT_IN_TEST_PREVIOUS_MAJOR:  10
(OPT_IN_TEST_NEXT_MAJOR:     currently disabled
 OPT_IN_TEST_PREVIOUS_MINOR: currently disabled)

Merged, and back to active for the other tasks (non-urgent).