integration test for trac #2951

add a test for the case which the db is too slow to complete the newLog before end of the step

if I revert previous commit, I get the error reported by benallard

===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/Users/ptardy/dev/bb/buildbot-heroku/buildbot/master/buildbot/process/buildstep.py", line 157, in next
    self._catchup()
  File "/Users/ptardy/dev/bb/buildbot-heroku/buildbot/master/buildbot/process/buildstep.py", line 159, in _catchup
    self.step._start_unhandled_deferreds.append(d)
exceptions.AttributeError: 'NoneType' object has no attribute 'append'

buildbot.test.integration.test_custom_buildstep.RunSteps.test_OldStyleCustomBuildStepSlowDB
buildbot.test.integration.test_custom_buildstep.RunSteps.test_OldStyleCustomBuildStepSlowDB
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/Users/ptardy/dev/bb/buildbot-heroku/buildbot/master/buildbot/process/buildstep.py", line 135, in gotAsync
    self._catchup()
  File "/Users/ptardy/dev/bb/buildbot-heroku/buildbot/master/buildbot/process/buildstep.py", line 159, in _catchup
    self.step._start_unhandled_deferreds.append(d)
exceptions.AttributeError: 'NoneType' object has no attribute 'append'

buildbot.test.integration.test_custom_buildstep.RunSteps.test_OldStyleCustomBuildStepSlowDB

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>