Offer fail-fast functionality

[vemv]

Hi there!

I found that it's easy to build "fail-fast" functionality on top of circleci.test.

The trick is changing this for:

https://github.com/circleci/circleci.test/blob/8bc3ed6ee364dce5d446b0b1163e7041a5149f7b/src/circleci/test.clj#L190-L191

..with a reduce that observes the return value of each iteration. If at any given iteration the :fail or :error count is > 0, then one can shortcircuit the loop with (reduced).

Would love to PR this, but I currently can't... ⚖️

Thanks - V