spec: speed up some model specs

reduce runtime of these files ~25% (3:54 -> 2:56)

Change-Id: Ibdb7260ac55c7272c8e35961fcfcdafa8da638ea
Reviewed-on: https://gerrit.instructure.com/38665
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>