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
1 file changed