The generational fuzzer test suites (and future FSM suites) can benefit from using the same model as the json schema tests of loading a JS test runner and passing in test files through TestData that the test runner can then use.

The main change is to rename the JSONSchemaTestCase (and related code) into something more generic, like JSRunnerTestCase. I think the only piece of logic change that is required is to have TEST_RUNNER_FILE be passed in, through the suite configuration, instead of being hard-coded.