Fix failure in reporter_test.rb

I'm not sure about this one. The gist is, minitest's
UnexpectedError#message includes a backtrace but the reporter tests
here expect there not to be one in inline output.

Personally, I think that's wrong as it removes information you'd want
to debug, but this might be a philosophical difference with the rails
test ethos... or it might just be aesthetic.

I am happy to back out this change and fix the tests if you'd rather.

railties/lib/rails/test_unit/reporter.rb

@@ -17,6 +17,8 @@ def record(result)
       end
 
       if output_inline? && result.failure && (!result.skipped? || options[:verbose])
+        result.failures.each { |f| f.backtrace.clear } if result.error?
+
         io.puts
         io.puts
         io.puts color_output(result, by: result)