Ensure unique path for all tmp directories in tester

This MR fixes a small bug introduced in !1073 (merged).

When multiple tests are defined with the same base name, but located in different directories the Tester creates a temporary directory using only the base name resulting in multiple tests with the same tmp directory. When the tests are run in parallel, these get clobbered and tmp directories get deleted while tests are running in them.

The fix implemented here simply adds a sequence of random characters to the end of the tmp directory name to ensure uniqueness.