Port test_accumulators.clj to cljc

[WilliamParker]

The name of the issue basically describes this; we should move the tests in test_accumulators.clj to a cljc test file that uses def-rules-test and thus is run in both Clojure and ClojureScript. Test_accumulators.clj focuses on tests of the accumulators supplied by Clara, as opposed to tests of the ability of Clara's engine to support accumulators.

[WilliamParker]

#312 was merged to address this; closing.