Fix the random test failure on ChatTest

So I ran the test many times in a row to confirm the issue locally, and looking at the error message I had a suspicion.
But I traced it down to see what is happening. And the reason it sometimes fails but not others is that we use $this->randomString() in the parts where this fails. The failure comes from the fact that we use ECA tokens, or more precisely tokens with a \Drupal\eca\Plugin\DataType\DataTransferObject, this in turn gets turned into a string by using Yaml::encode($values). But some random strings get changed by that other than just getting surrounded by '.
The example in the issue summary is KO"'>&b( which becomes 'KO"''>&b(' and that in turn does not contain the original.
This can happen in all the test methods that use the ECA token replacement, so I fixed it by fixing the randomString method in the base class. I ran tests 250 times and they succeeded every time. But we can use another approach too if this is deemed not to be the right approach.

Looks good to me.

Thank you bircher, getting merged.