tests: fix sss_nss_idmap-tests
    
    If sss_nss_idmap-tests is complied with -Wl,-Bsymbolic-functions it
    fails because sss_nss_make_request_timeout() is not properly wrapped but
    the tests expects that the replacement call it provides is picked by the
    linker and not the original function.
    
    This patch wraps the call in the corresponding test library. As a
    consequence a small helper function had to be added to make dlopen-tests
    pass for the test library as well.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3801
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Tested-by: Andreas Hasenack <andreas.hasenack@canonical.com>