Need to make some blocks/ tests

[jpolitz]

It would be useful to make some Selenium tests for /blocks, at least loading the page, figuring out what to wait on, running a simple program, etc. Probably needs a few new helpers in test-util to recognize and run things on that page, and maybe some clever ways to populate blocky programs.

[jpolitz]

(@ds26gte and I just got a green build on a silly configuration error about starter vs. essentials in blocks. We caught it manually testing, but would be nice to not rely on that!)