avocado: Make tests to refer their ids

Make avocado tests to refer to their test ids (paths),
relative to the directory you are in. For example,
running:

avocado run tests/sleeptest.py

will give you

DEBUG LOG: /home/lmr/avocado/logs/run-2014-07-31-12.54.01/debug.log
TOTAL TESTS: 1
(1/1) tests/synctest.py.1: PASS (1.93 s)

Test log and data directories were changed accordingly. Now,
if your test is synctest.py, your data dir will be called
synctest.py.data.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>