This change restructures the test framework code a bit, to make it
easier to introduce additional languages. Our knowledge of Yarn and
Node.js project structure, for instance, was previously baked in to
the test logic, in a way that was hard to make, for instance, Yarn
optional. (In Python, of course, it will not be used.) To better
support this, I've moved some state onto a new programTester struct
that we can use to lazily find binaries required during the testing
(such as Yarn, Pip, and so on). I'm committing this separately so
that I can minimize merge conflicts in the Python work.
