refactor(tests): fix waiting list reminder assertion

The delivery-assertion refactor incorrectly asserted that already-reminded

invitations should have reminded_at as nil. The fabricator sets

reminded_at to 2.days.ago, so the correct assertion is that it

remains unchanged (still ~2 days ago).

Author: mroderick

spec/models/invitation_manager_spec.rb

@@ -156,7 +156,9 @@
       }.to change { ActionMailer::Base.deliveries.count }.by(invitations.count)
 
       invitations.each { |invitation| expect(invitation.reload.reminded_at).not_to be_nil }
-      reminded_invitations.each { |invitation| expect(invitation.reload.reminded_at).to be_nil }
+      reminded_invitations.each do |invitation|
+        expect(invitation.reload.reminded_at).to be_within(1.second).of(2.days.ago)
+      end
 
       emails = ActionMailer::Base.deliveries.last(invitations.count)
       invitation_emails = invitations.map { |i| i.member.email }