Correct plugin test location reporting

Commit 6f67e3787338407e637d80a6fad0edb5f9a08b24 changed the
directory structure for the plugin, but the code that reports
where the tests can be found was not fully updated.

This updates the plugin reporting to get the correct directory
for tempest to look in and cleans up a few nits with the file.

Change-Id: Ib89f7a9493e3d83382d8dd1f07a3e9e4bb4fcd80
Closes-bug: #1745175
1 file changed