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>
(cherry picked from commit 6411cd6074688762f793de8f1dddeffcb3a71d02)