build: add circleci test reporting

[erickzhao]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This PR uses the store_test_results step in our CircleCI config. To collect test results, I added the mocha-junit-reporter package. This reporter will run when process.env.CI is true.

This is a pre-requisite for using Circle's "split by timing" test parallelism feature: https://circleci.com/docs/parallelism-faster-jobs/

This PR follows the Collect Test Data doc from CircleCI.