Use assert_not in bug report template TestCase

`assert_not` is preferred instead of `refute`, according to the Rails Guide:

https://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#follow-the-coding-conventions

Author: jeromedalbert

guides/bug_report_templates/generic.rb

@@ -17,6 +17,6 @@
 class BugTest < ActiveSupport::TestCase
   def test_stuff
     assert "zomg".present?
-    refute "".present?
+    assert_not "".present?
   end
 end