Offer fail-fast functionality

Hi there!

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

The trick is changing this for:

Lines 190 to 191 in 8bc3ed6

    
           #(assoc (apply merge-with + (for [n nses] 
        
                                         (test-ns n selector config)))

..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

	#(assoc (apply merge-with + (for [n nses]
	(test-ns n selector config)))