Fix plugin.py test directory

Fixes plugin.py test directory by correcting the path
in ``test_dir`` to be "patrole_tempest_plugin/tests/api"
where the Tempest-based API tests are located.

ostestr --list | grep patrole should now only list API tests,
and exclude unit tests from the list (which previously
appeared).

Change-Id: I2f4bac8296092092a1b6a8d36f5ac622f933833f
Closes-Bug: #1691145
diff --git a/patrole_tempest_plugin/plugin.py b/patrole_tempest_plugin/plugin.py
index 3abf4aa..cfe5c0a 100644
--- a/patrole_tempest_plugin/plugin.py
+++ b/patrole_tempest_plugin/plugin.py
@@ -25,7 +25,7 @@
     def load_tests(self):
         base_path = os.path.split(os.path.dirname(
             os.path.abspath(__file__)))[0]
-        test_dir = "patrole_tempest_plugin/tests"
+        test_dir = "patrole_tempest_plugin/tests/api"
         full_test_dir = os.path.join(base_path, test_dir)
         return full_test_dir, base_path