Support option to opt out of terminating on any failures

[crisptrutski]

When chaining test runners together (eg. unit tests and in-browser scenario tests), it can be more illuminating to get all failures at once, not only those from the first runner to have failures.

It also seems useful to be able to hang onto the exit code until later - so that boot test test-cljs would still be able fail on CI given only boot-test failures.

Towards this end I'm considering an exit! task for boot-cljs-test, which hangs on to any prior failures. Crude, but perhaps workable with some amendments, can be seen in crisptrutski/boot-cljs-test#24