Fix sychronization issues in LTI lettuce test

@benpatterson 
This looks like it fixes the failures on Solano CI also.

* Fix sychronization issues in LTI lettuce test
* Better verification for troubleshooting LTI test failures
* Dismiss any lingering alerts after lettuce scenarios
I'm guessing not now, but wanted to ask: do we need to worry about >1 alerts being open?