fix flaky assignment specs

0 means 0, not "approximately 0" for caching role override stuff

Change-Id: Ic3b3cf44b8477fd6f61754fa31c51885a38b0b62
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/244679
Reviewed-by: Aaron Ogata <aogata@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

lib/fallback_memory_cache.rb

@@ -18,6 +18,11 @@
 class FallbackMemoryCache < ActiveSupport::Cache::MemoryStore
   KEY_SUFFIX = '__no_expire'.freeze
 
+  def fetch(*, expires_in: nil)
+    return yield if expires_in == 0
+    super
+  end
+
   def fetch_without_expiration(key)
     fetch(key + KEY_SUFFIX)
   end