Add start directory for oslo_debug_helper

When I used `tox -e debug <test-name>` to test a case, the case did not
run properly and there is an error like this:

ImportError: Start directory is not importable: './python-watcher/tests'

If we use '-t wartcher/tests' to point out the test path, the case will
run properly under debug.
So we'd better add this start directory for oslo_debug_helper.

Change-Id: I04d9937f72a95f8f045129af08df0cd0d0870d39

tox.ini

@@ -35,7 +35,7 @@ commands =
     python setup.py build_sphinx
 
 [testenv:debug]
-commands = oslo_debug_helper {posargs}
+commands = oslo_debug_helper -t watcher/tests {posargs}
 
 [testenv:config]
 sitepackages = False