[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 doesn't work if the dir is not empty. You could use `Utils.createTempDir()` and, optionally, `Utils.deleteRecursively()` (since `createTempDir` already takes care of that for you).