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>
Oh, I see that my point was dubious. I see that a test may not get a job, but I actually meant that I can't see how a job won't have a unique_id attribute.

If my (now hopefully clearer) point is valid, then that code could become:

```
if job is not None:
   tmpdir = tempfile.mkdtemp(dir=data_dir.get_tmp_dir(),
                                      prefix='job-%s-' % job.unique_id)
else:
   tmpdir = tempfile.mkdtemp(dir=data_dir.get_tmp_dir())
```