spec: once-ler spec/models/{b,c}*

in rails3, reduce runtime by >25% (25.5min -> 18.5min locally)

Change-Id: If2980f721f59fc04cff8d77857bb8554190c1935
Reviewed-on: https://gerrit.instructure.com/37072
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>