spec: don't truncate cassandra in tests

Turns out that truncate is a super slow operation in Cassandra. For the
tests that this affects, we'll handle the potentially non-empty db.

Change-Id: Ie7e64add3b53f26eb98ba441e46270a8c08657ea
Reviewed-on: https://gerrit.instructure.com/36593
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Shawn Meredith <shawn@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>