.travis.yml: Add some functional testing of avocado

Let's actually execute avocado in scenarios where we
know what the produced outcome is:

* Execute a list of tests that pass, expected rc == 0
* Execute a list of tests where some fail, expected rc == 1

Although very simple, it's already a good start of
functional testing of the code during each travis job.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #44 from avocado-framework/expand-travis-testing

.travis.yml: Add some functional testing of avocado