spec: refactored admin_settings_spec.rb for CNVS-16302

Change-Id: I57155f1e1438b17a5fa375a3557625dfe189752f
Reviewed-on: https://gerrit.instructure.com/42875
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>