Improve XDebug integration for tests and run-tests.sh

Running for d10.1

Tests pass fine. Not sure what more can be tested.

As per 🐛 document run-tests.sh as not intended for public consumption Fixed run-tests.sh is only really intended for DrupalCI - it is easier to run tests locally by directly invoking PHPUnit, so I am not sure that this should be added. The new switch isn't documented either.

Marking needs review for further discussion, but I suspect this should be closed (won't fix).

@longwave, I thought the same as you, that this is won't fix. In fact, I thought I left a comment. Clearly, that was wrong.

Will close out then and lean on your judgement for this.

If anyone disagrees please reopen explaining why.

Thanks.