GraphX unit tests fail nondeterministically

GraphX unit tests have been failing since the fix to SPARK-2823 was merged: https://github.com/apache/spark/commit/9b225ac3072de522b40b46aba6df1f1c231f13ef. Failures have appeared as Snappy parsing errors and shuffle FileNotFoundExceptions. A local test showed that these failures occurred in about 3/10 test runs.

Reverting the mentioned commit seems to solve the problem. Since this is blocking everyone else, I'm submitting a hotfix to do that, and we can diagnose the problem in more detail afterwards.