[SPARK-][Build] Cleans lib_managed before compiling with Hive 0.13.1

This PR tries to fix the Hive tests failure encountered in PR #3157 by cleaning `lib_managed` before building assembly jar against Hive 0.13.1 in `dev/run-tests`. Otherwise two sets of datanucleus jars would be left in `lib_managed` and may mess up class paths while executing Hive test suites. Please refer to [this thread] [1] for details. A clean build would be even safer, but we only clean `lib_managed` here to save build time.

This PR also takes the chance to clean up some minor typos and formatting issues in the comments.

[1]: https://github.com/apache/spark/pull/3157#issuecomment-67656488