Add a `cleanUp` argument to the `FlakyTest.retry` combinator

As evinced by the recent failures of the `pr-scala-integrate-ide` job, before
re-running a test we need to make sure that we clean-up any side-effect created
by the previous test run.
Merge pull request #566 from dotta/wip/add-cleanup-to-flakytest-retry

Add a `cleanUp` argument to the `FlakyTest.retry` combinator