avocado.test: Use job.unique_id to build temporary path.

if job.unique_id is available in test, use this information to build the temporary path used in attribute workdir. The template format is `$TMP/job-<job_unique_id>-<random>`.

Here's an example:

<pre>
$ ./scripts/avocado run tests/synctest.py tests/sleeptest.py --keep-tmp-files
...
$ find /var/tmp/avocado
/var/tmp/avocado
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg/synctest.py
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg/synctest.py/src
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg/synctest.py/src/synctest
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg/synctest.py/src/synctest/synctest
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg/synctest.py/src/synctest/synctest.c
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-gmwbDg/synctest.py/src/synctest/Makefile
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-tZG3kV
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-tZG3kV/sleeptest.py
/var/tmp/avocado/job-d7c3acdcabc184990dfa90b69c313a19e12c2c31-tZG3kV/sleeptest.py/src
</pre>

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Just as another point, I ran the selftests with the proposed change, and the results are good.

```
./run all 
...........................................................................................................................S.S...........
----------------------------------------------------------------------
Ran 135 tests in 177.009s

OK (SKIP=2)
```