avocado: Add NOT_FOUND test status

To avoid confusion, when passing an invalid test ID
(typo, incorrect path, alias that could not be resolved),
introduce a new status, NOT_FOUND inside avocado, as
well as proper handling handling for such situations.

    $ scripts/avocado run sbrubles
    JOB ID    : 312b690173a8f5f261d9c5cefdb8699ac0798ded
    JOB LOG   : /home/lmr/avocado/job-results/job-2014-08-14T10.02-312b690/job.log
    TESTS     : 1
    (1/1) sbrubles: NOT_FOUND (0.00 s)
    PASS      : 0
    ERROR     : 0
    FAIL      : 0
    SKIP      : 0
    WARN      : 0
    NOT FOUND : 1
    TIME      : 0.00 s

Also, add a functional test for this new status.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>