Workaround unsupported syntax in MRI tests

Author: eregon

test/mri/tests/error_highlight/test_error_highlight.rb

@@ -1207,7 +1207,7 @@ def test_simulate_funcallv_from_embedded_ruby
 
       nil.foo + 1
     rescue NoMethodError => exc
-      def exc.backtrace_locations = []
+      def exc.backtrace_locations; []; end
       raise
     end
   end