spec: disable cassandra table truncation spec_helper

Change-Id: I16d6a9bfaeb35f07c1faaa3051ad9753d3103bf7
Reviewed-on: https://gerrit.instructure.com/36592
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>