Avocado tests: use children loggers of "avocado.test"

Avocado has changed behavior in the sense that it does not change
interpreter wide logging configuration for namespaces outside of the
"avocado" namespace.

Because the Avocado test already contains the "avocado.test" logger as
an attribute at "self.log", let's use children loggers for other test
related logging.

Signed-off-by: Cleber Rosa <crosa@redhat.com>