spec: don't allow requests to span spec boundaries

ensure any in-flight requests (ajax or otherwise) finish processing before
we wrap up each spec, and prevent them from triggering additional requests.

this fixes various race condition failures around our shared db conn, e.g.
PG::ForeignKeyViolation, PG::InFailedSqlTransaction, as well as the
occasional segfault

Change-Id: Idfed1244a921f7742acc84b03fe33e3084f03568
Reviewed-on: https://gerrit.instructure.com/44501
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>