fix for ticket:2951: wait for the logs are created before finishing the step

There is a race condition between addLogs calls, and the end of the step.

for oldStyleSteps, this matters, because the subsequent addStderr, are only called
at the end of the step, so we must wait for log are created before continuing

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
I love the fix, but this has me worried because it's using clock time to determine which side of a race wins.  Could you add a hook at the beginning of `finally` block run `run`, and use that hook to call `reactor.callLater(0, delayed)` in the tests?  That would mean that `newLog` doesn't return until the reactor iteration after the `yield` statement.