add tests for missed PrematureStops
add tests for missed PrematureStops

Called Silent but should be PrematureStop
7 117182143 . T TGTA . . . GT 0/1
Called Deletion but should be PrematureStop
7 117182143 . TAACAG TATAAC . . . GT 0/1
Called Insertion but should be PrematureStop
7 117182143 . TA TATAGGTT . . . GT 0/1

hammerlab/varcode,hammerlab/varcode