Flat tests

Make avocado tests flat (not needing directories), adapt everything to the new rules.
In the case of some python module foo.py has no Test class, the test will fail with a message like this:
<pre>
./scripts/avocado run foo.py
DEBUG LOG: /home/rmoura/avocado/logs/run-2014-07-29-18.04.59/debug.log
TOTAL TESTS: 1
Avocado crashed: UnboundLocalError: local variable 'test_class' referenced before assignment
Traceback (most recent call last):

  File "/home/rmoura/Work/avocado.git/avocado/job.py", line 341, in _run
    failures = self.test_runner.run(params_list)

  File "/home/rmoura/Work/avocado.git/avocado/job.py", line 148, in run
    test_instance = self.load_test(params)

  File "/home/rmoura/Work/avocado.git/avocado/job.py", line 99, in load_test
    test_instance = test_class(name=module_name,

UnboundLocalError: local variable 'test_class' referenced before assignment

Please include the traceback info and command line used on your bug report
Report bugs visiting https://github.com/avocado-framework/avocado/issues/new
</pre>

How about to protect Avocado from a case like this, telling that no test  was found?