Merge multiple functional tests together to reduce test startup time

[mstenta]

I was looking into ways of improving the performance of our functional tests, and found this suggestion by @Berdir:

Another thing to consider is to avoid having many test methods. Each runs in a completely new, isolated environment. A trick that is often used is to have a single testAll() method and many doTestSomething() methods, then they are faster by almost factor count(test methods), since set up is usually by far the slowest part.

https://drupal.stackexchange.com/a/213561

This PR tests out this suggestion. It seems to shave off about 30 seconds from our tests overall, based on the few times that I've run them so far.

[Berdir]

I have been summoned ;)

FWIW, consider whether this is actually worth. It also reduces test isolation, which might cause weird bugs in tests and makes them harder/slower to run when debugging a specific test, you need to comment out things or so or you also always need to run a bulk of tests.

[mstenta]

Thank you for your input and insight @Berdir, and for taking the time to offer it here! 😄 🙏

These are great considerations. I agree we should weigh the costs of doing something like this. I was curious how big a difference it would make. In the process I also discovered some tests that clearly did not need to be separate, so it was a good exercise. Perhaps that would be a good next step: look at the tests that were modified and decide on a case by case basis if there is value in keeping them separate.