add specs for variable substitutions

Test Plan:
 - specs should pass

Change-Id: I4267d457f2f58c9913e36ff9f59cf8e9dbc96db6
Reviewed-on: https://gerrit.instructure.com/40315
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>