The function "add_console_handler" is quite useful. This patch extends
it's capabilities and renames it to fit the new purpose.

Because the new default logging level for "add_log_handler" is INFO,
we add the "avocado.test" and root logger to level DEBUG.

Then finally a few adjustments to the tests that rely on specific
log format and level:

1) selftests/functional/test_multiplex.py: adapt to changes in
test logger That means that extra prefixes ('avocado.test') are
printed out on each line when avocado is run with '--show-job-log'.
While at it, be more precise and check for the specific params at the
specific run with the given variant, instead of looking for a given
param in the output of the three tests variants.

2) selftests/functional/test_standalone.py: adapt to the change of
log prefix ('avocado.test') and extra line because of DEBUG level.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>
Signed-off-by: NCleber Rosa <crosa@redhat.com>