For these test suites, shouldn't the [afterEach() method in the LocalSparkContext trait](https://github.com/AustinBGibbons/MLI/blob/sc-stop/src/test/scala/testsupport/LocalSparkContext.scala?pr=%2Famplab%2FMLI%2Fpull%2F5#L16) take care of stopping these contexts?  SparkContext.stop() is idempotent and an extra call wouldn't cause problems, though.