spec: close open dialogs before starting LTI dialog spec

Change-Id: I97385ab3c1ff337a6c289dfe50b2ba8bcbea2001
Reviewed-on: https://gerrit.instructure.com/35630
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>