The test server's flow handler awaited flowPOST before incrementing the
invocation counter, which races with the test polling getRun() to see
the completed status. When the workflow completed inside flowPOST and
flushed the run to the DB, the test could observe the completed state
and immediately query /_flow-invocations before the counter was bumped,
yielding a flaky 'expected 0 to be 1' assertion.
Increment the counter before awaiting flowPOST so the count is
observable as soon as the run transitions to completed.
Nathan Rajlich