Add a new property to ProgramTestOptions, `Overrides` that allows a
test to request a different version of a package is used instead of
what would be listed in the package.json file.
This will be used by our nightly automation to run everything "at head"
Because we run our golang integration tests "in tree" (due to
the need to be under $GOPATH and have a vendor folder around), the
"command-output" folder was getting left behind, dirtying the worktree
after building.
This change does two things:
1. On a succecssful run, remove the folder.
2. Ignore the folder via .gitignore (this way if a test fails and you
do `git add .` you don't end up commiting this folder).
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.
