Add support for overriding test main_class

[SevereOverfl0w]

This allows doing this:

(ns griffin.test.runner
  {:bazel/clojure_library {:deps ["@rules_clojure//src/rules_clojure:testrunner"]}}
  (:require [rules-clojure.testrunner :as testrunner])
  (:gen-class))

(defn -main
  [& args]
  (binding [*print-length* 1]
    (apply testrunner/-main args)))

by setting this in your clojure_test deps.edn alias:

:clojure_test {:main_class "griffin.test.runner"
               :deps ["//src/griffin/test:runner"]}