[SPARK-4826] Fix generation of temp file names in WAL tests

This PR should fix SPARK-4826, an issue where a bug in how we generate temp. file names was causing spurious test failures in the write ahead log suites.

Closes #3695.
Closes #3701.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3704 from JoshRosen/SPARK-4826 and squashes the following commits:

f2307f5 [Josh Rosen] Use Spark Utils class for directory creation/deletion
a693ddb [Josh Rosen] remove unused Random import
b275e41 [Josh Rosen] Move creation of temp. dir to beforeEach/afterEach.
9362919 [Josh Rosen] [SPARK-4826] Fix bug in generation of temp file names. in WAL suites.
86c1944 [Josh Rosen] Revert "HOTFIX: Disabling failing block manager test"