t2-run-workflow: Cope with hitting max running runs.

Informs the user and deletes the run for now.
t2-server-stress: Cope with hitting max running runs.

Backs of for 10 seconds before retrying if the server doesn't start a run due to
overcapacity. As this is a script to stress test then it is appropriate to wait
and continue running. This keeps the server at full capacity within its
configured limits.
Ensure all unit tests delete runs that they create.

Some tests were not deleting runs. This was being hidden by the fact that the
last test run was (randomly, so a coincedence) one that tested
Server#delete_all_runs.

Tests now run correctly if the run limits of both existing and executing runs
are set to 1.