LP #1536623: Basic test cases should be created for PAM plugin

**Reported in Launchpad by Laurynas Biveinis last update 21-01-2016 13:59:28

We have avoided testing PAM plugins in the past in MTR environment, as performing actual authentication requires system-wide policy configurations. But e.g. scenario of bug 1535631 suggests that some minimal MTR testing (install plugin, create user) without authentication attempts would be beneficial even if not providing enough coverage for the whole of plugin functionality.