Starts one occurrence of a saved schedule now and returns the new test run.
Use the returned id to follow the run, cancel it, and get its results.

• {id} is a scheduler ID from the schedule's schedulers[], not the
  schedule ID.
• startTime is that scheduler's nextTime. It selects the occurrence; the
  run starts immediately. A time in the past is rejected.

Requires the TestRun:Create permission. If the request times out, check
the test-run list before you retry: the run may already exist.