Improve tests on Artifactory backend by open the connection only one time

[hagenw]

As shown in #215 actions on the Artifactory server are much slower if we authenticate every time we do something on the server. Instead we can authenticate one time, and reuse the returned path object to do all the relevant stuff.

#215 adjusts the code of audbackend.backend.Artifactory accordingly, but for the tests we still do a new connection with every instance of the interface fixture. We should change this as well.