Using more than one docstrig directive makes Avocado to use only the
first one. This patch makes the safeloader to handle multiple docstring
directives (i.e. `🥑 enable` and `🥑 tags=foo`).

Reference: https://trello.com/c/6FpySXuCSigned-off-by: NAmador Pahim <apahim@redhat.com>

The unittest standard library on Python 2.7 an later has everything
that the unittest2 backport is supposed to have.  Let's then drop all
the conditional imports of unittest2 and stick with unittest.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

This adds utility functions to parse class-level docstring contents
with a "tags" directive.

The idea is that test writers should be able to categorize their tests
and test runners should be able to further filter the result of the
test reference resolution using that information.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Commit 2adc953e changed how we used to refer to "avocado tags", and now
we call them "docstring directives".  Let's do that in the code too,
so that our users and hackers (and ourselves) have a more consistent
experience.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

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

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

The idea is to hold utilities that are used to implement the safe,
AKA ast-based, test loader.
Signed-off-by: NCleber Rosa <crosa@redhat.com>