Allow multiple constrained random answers tables

[plocket]

The feature for generator files (#962) is almost done and at the moment authors are only allowed to have one constrained random answers Step/table per generator Scenario. I just realized we could go about this a slightly different way, though I'm not yet sure how to clarify the language for the authors.

The thought has been that each constraints Step itself shows how many number of tests to generate. Having multiple Steps like that would cause exponential growth in the number of tests to generate and is notoriously hard for humans to grok on a deep level.

The new idea: A generator Scenario could have one Step that described the number of tests to generate that is separate from the constraints Steps themselves. ALKiln would randomize answers for the Scenario as a whole and stop when it reached the requested number of tests.

This requires re-working the generator Scenarios, but it may be better to do that now than to wait till after we release the first, experimental, version. That said, the current version is almost ready to go.

[plocket]

This change is probably a good idea. I considered whether it was necessary or not, but it seems likely to be necessary because of these complications:

• Some interviews have background actions they have to explicitly wait for until we can implement waiting for those (if it's even possible).
• Some questions may need to set values with GitHub secrets, which Story Tables can't yet do.
• Authors might want to observe things about intermediate pages, like taking a screenshot or finding a phrase.
• Authors will be unable to double up on the functionality of their tests. For example, they won't be able to check an invalid path and then resume the interview to get to the end.

This proposed behavior may not solve those problems completely. For example, interviews that go down one path might need to skip the page that sets the GitHub secret while another path may need to go through that page. Authors can make 2 types of generator Scenarios for each of those eventualities, but it's awkward. I'm not sure how else we can advise authors right now.