TESTS: Add a module to call nss_sss's getpw* from tests
    
    Implements a python module that allows to load the nss_sss module and
    simulate calling getpw* functions from tests.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>