Make :migration specs a little more robust

E.g. https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/52873034

spec/spec_helper.rb

@@ -154,6 +154,22 @@ RSpec.configure do |config|
     Sidekiq.redis(&:flushall)
   end
 
+  # The :each scope runs "inside" the example, so this hook ensures the DB is in the
+  # correct state before any examples' before hooks are called. This prevents a
+  # problem where `ScheduleIssuesClosedAtTypeChange` (or any migration that depends
+  # on background migrations being run inline during test setup) can be broken by
+  # altering Sidekiq behavior in an unrelated spec like so:
+  #
+  # around do |example|
+  #   Sidekiq::Testing.fake! do
+  #     example.run
+  #   end
+  # end
+  config.before(:context, :migration) do
+    schema_migrate_down!
+  end
+
+  # Each example may call `migrate!`, so we must ensure we are migrated down every time
   config.before(:each, :migration) do
     schema_migrate_down!
   end