New feature: silent (human) test result.

Implement new option --silent, to mute the human test result.
It has no effect with --json or --xunit options.

Example:

$ avocado --silent run failtest
$ echo $?
$ 1

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #141 from lmr/silent_output

New feature: silent (human) test result.
avocado.job: Give an error message on an empty test list

If an empty list of tests was passed, give an error validation
message and exit. Provide a functional test for it.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #142 from lmr/empty-test-list

avocado.job: Give an error message on an empty test list
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>
Merge pull request #143 from lmr/test-not-found

avocado: Add NOT_FOUND test status