Move code over to new siteconfig_settings context manager.
Review Request #10184 — Created Sept. 26, 2018 and submitted
release-4.0.x had added an
override_siteconfigcontext manager for unit
tests, and then later release-3.0.x got a very similar one using the more
siteconfig_settings. This change moves the former
over to use the latter.
Ran unit tests.