spec: fix missing sharding_spec_helper requires

These files were only working before because an always-loaded file was
requiring the helper for them.

Change-Id: I1704d998ebfdc100455dfa63d37a30827664288d
Reviewed-on: https://gerrit.instructure.com/43323
QA-Review: Shawn Meredith <shawn@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Shawn Meredith <shawn@instructure.com>
Reviewed-by: Shawn Meredith <shawn@instructure.com>