run-test.sh outputs diffs in stable order
run-test.sh does not diff things twice

exit code is affected since it is a sum of exit codes from
diff(1) for all X.{actual,expected} pairs.
Merge branch 'stable-order-exitcode'

* stable-order-exitcode:
  run-test.sh does not diff things twice
  run-test.sh outputs diffs in stable order