Include `testing-contexts` names in junit output

[glenjamin]

At the moment the unit output doesn't include the subtitles from (testing) calls at all.

This makes is tricky to see which test is failing in blocks with multiple scenarios.

[gordonsyme]

Started on in https://github.com/circleci/circleci.test/pull/34 already.

[glenjamin]

Oh cool, I searched open issues, but evidently not PRs.

[gordonsyme]

The motivation to pick it back up again is good, I just saw the timestamp on that PR