There are few issues (3 to my count) which affect a number of tests
on Python 3.  Instead of not executing any of the functional tests,
let's skip the broken ones.

This is important because it will prevent new issues from making
its way into the Avocado code.

The list of tests being skipped is being tracked on our planning board,
so that they are accounted for, and enabled when fixed.

Reference: https://trello.com/c/eFY9Vw1R/1282-python-3-functional-tests-checklistSigned-off-by: NCleber Rosa <crosa@redhat.com>