pam-srv-tests: split pam_test_setup() so it can be reused
    
    Split pam_test_setup() so domain and pam parameters can be easily set
    distinctly for each test.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2697
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>