fix finding pre-existing test shards

Change-Id: Idf6c59c3b90beda74801750be0b3c462ff8ed292
Reviewed-on: https://gerrit.instructure.com/28708
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>