Add tooling for testing dependencies

When we upgrade our dependencies, particularly when we upgrade to versions that aren't explicitly tagged as a release, we don't know for sure that the version we are getting is a good one. As a step to partially vet the quality of the commits we upgrade to, we have language in CONTRIBUTING.md that requires pull requests document the tests they ran, and that reviewers verify this.

However, it's pretty hard to do this consistenly and thoroughly, especially in the Go modules world. Also, the test output is quite unwieldy in PRs and commit messages. We should write some tooling that either tests all packaged currently vendored, or figures out what has changed vs master, and tests those packages.

Start by reading CONTRIBUTING.md and reviewing how this repository manages Go modules and currently vendored packages. Define whether the tooling tests all vendored packages or only packages changed versus master, then establish how concise test results should be reported. Done means the selected dependency-testing workflow can be run consistently and produces usable output for pull requests or commit messages.