Changes

testenv: improve --config argument parsing

* Fix that it didn't complain about and invalid --config argument, as
  long as there was a valid --config argument before it.
* Let raise_error_config_arg only output the invalid --config argument
  instead of all of them.
* Complain if "--config all" is used in combination with another
  --config argument.

Change-Id: I66b976b0332be523c084a6b5d38d0f62134b495d