To be honest, our test code could still be kept in the same directories and
have the same names. But I think we can improve two things here:

1) Flatten a little bit the directory structure of selftests. Two path
components are being dropped here: "all" and "avocado". So that
"selftests/all/functional/avocado" becomes simply
"selftests/functional".

2) File names match what is, by default, recognized by unittest (the
Standard Library module) based discovery of tests. That means that
doc_build_test.py becomes test_doc_build.py. Not a big deal IMHO.
Signed-off-by: NCleber Rosa <crosa@redhat.com>