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>
Looks like this shouldn't be needed, as we have in `Job.__init__`:

```
        if args is not None:
            self.unique_id = args.unique_id or job_id.get_job_id()
        else:
            self.unique_id = job_id.get_job_id()
```

or maybe I'm missing something.