Test tags (via docstring directives) have been supported in Avocado
for a number of releases, but up until now user would only be able to
see the tags on their tests by resorting to the source code.

This was already difficult when the tags where set only at the class
level, now that they can be set both on the class and method
docstring, users would have to do the "math" on their minds.

Let's display the test tags when the verbose switch is given to the
list command to make the life of users easier.
Signed-off-by: NCleber Rosa <crosa@redhat.com>