Use /taskflow/flush-test in the flush function

Instead of using /tmp use a taskflow specific
directory for the flush testing utility function
to make it possible to cleanup any leftover taskflow
test directories if/when a test crashes or fails without
cleaning itself up.

Change-Id: I4d06adf9b9fbf9ec70b511e763edb52a41293e39
Merge "Use /taskflow/flush-test in the flush function"