spec: fix flakey groups_teacher_spec

refs DE-12

flag = none

Change-Id: Id8ef710f99786c23bed558c8f8006bac8ed6b8dd
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/240316
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: James Butters <jbutters@instructure.com>
QA-Review: Aaron Ogata <aogata@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>

spec/selenium/groups_teacher_spec.rb

@@ -163,6 +163,7 @@ describe "new groups" do
       f(".message-all-unassigned").click
       replace_content(fj('textarea[name="body"]'), "blah blah blah students")
       fj(".btn-primary[data-text-when-loaded='Sent!']").click
+      wait_for_ajaximations
 
       expect(@course).to eq Conversation.last.context
     end