pam-srv-tests: Add UT for cached 'online' auth.
    
    Extend PAM responder unit test to check 'online' cached authentication.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2697
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 4b12be504e20173e0629835818e4db6a9617a9a4)