spec: fix flaky spec in groups_teacher_spec.rb:157

closes: KNO-184

Another case where bug fixes have taken care of previous
flakiness. Unskipping.

Change-Id: I42594bc127f8ad51bbe739b1841b30e9ebea6c0f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/226442
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: August Thornton <august@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: August Thornton <august@instructure.com>

spec/selenium/groups_teacher_spec.rb

@@ -155,7 +155,6 @@ describe "new groups" do
     end
 
     it "should allow teachers to message unassigned students" do
-      skip('KNO-184')
       group_test_setup
 
       get "/courses/#{@course.id}/groups"