Run tests of kaocha plugins within main kaocha CI

[plexus]

Kaocha provides an API to plugins and test types, we should make sure we don't break that interface/contract. Currently we only notice when touching the plugin code, since they live in separate repos. We should run the tests of these plugins within Kaocha's CI run, so we notice any breaking changes.

[alysbrooks]

@plexus I think this is covered by #413, although it doesn't test kaocha-cljs or kaocha-cljs2 because running multiple ClojureScript test suites seems like it would be too flaky.