Exit code usage susceptible to wrapping

[frenchy64]

I'm working on fixing lein test's use of System/exit here.

I noticed that circleci.test has the same shortcoming in these places:

• circleci.test/src/circleci/test.clj

  Line 243 in 8bc3ed6

  (System/exit (+ (:error summary) (:fail summary))))))

• circleci.test/src/circleci/test.clj

  Line 253 in 8bc3ed6

  (System/exit (+ (:error summary) (:fail summary)))))

In a nutshell, I believe if a multiple of 256 failures+errors occurs in circleci.test, an exit code of 0 may occur.