The python unittests should be executable even now as a simple tests or
via external runner + contrib script. Let's create a new type and
discover python unittests as first-class test types in Avocado.

As Avocado tests are in fact python unittests, we need to manually skip
these by ignoring tests that are discovered as Avocado tests, but are
marked as not-to-be-executed, which required the additional change to
_find_avocado_tests method.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>