Add buildkite support - Test Analytics

[mfridman]

Buildkite recently added a new feature called Test Analytics.

Consider adding support for importing JSON-formatted test results to Buildkite, docs here:

https://buildkite.com/docs/test-analytics/importing-json
https://buildkite.com/docs/test-analytics/importing-json#json-test-results-data-reference

I've been working on something similar via https://gotest.io (private beta). So whatever user-facing method of setting flags/env variables should be consistent.