TESTS: add a helper module with shared NSS constants
    
    Every module that reads the sssd_nss module directly copied around the
    same definition of NSS constants. This commit moves them into a single
    file to avoid code duplication.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>