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

This PR is another approach for fixing 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.
This is kind of an unrelated change, but I wanted to remove this `Random.nextString()` call since it seemed confusing and didn't seem to serve any obvious purpose, since the `tempDir` is re-created before each test anyways.