avocado.job: Refactor code to detect conflicting use of stdout

Add a helper method on TestResultProxy() to get information
of output plugins using stdout, making up for a cleaner
way of letting the user know about the conflict.

Also, rephrase the error messages and update functional
tests to reflect the new status quo.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>