tests/acceptance: show avocado test execution by default

tests/acceptance: show avocado test execution by default

The current version of the "check-acceptance" target will only show
one line for execution of all tests.  That's probably OK if the tests
to be run are quick enough and they're always the same.

But, there's already one test alone that takes on average ~5 seconds
to run, we intend to adapt the list of tests to match the user's build
environment (among other choices).

Because of that, let's present the default Avocado UI by default.
Users can always choose a different output by setting the AVOCADO_SHOW
variable.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-2-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>