test-nss: do not assume all symbols are defined

It is OK for some symbols to be missing. With this change, "test-nss sss" can
be used to test nss-sss without crashing.

$ build-rawhide/test-nss sss fedoraproject.org
======== sss ========
_nss_sss_gethostbyname4_r not defined

_nss_sss_gethostbyname3_r not defined

_nss_sss_gethostbyname3_r not defined

_nss_sss_gethostbyname3_r not defined

_nss_sss_gethostbyname3_r not defined

_nss_sss_gethostbyname2_r("fedoraproject.org", AF_INET) → status=NSS_STATUS_NOTFOUND
                   errno=0/--- h_errno=-1/Resolver internal error

_nss_sss_gethostbyname2_r("fedoraproject.org", AF_INET6) → status=NSS_STATUS_NOTFOUND
                   errno=0/--- h_errno=-1/Resolver internal error

_nss_sss_gethostbyname2_r("fedoraproject.org", *) → status=NSS_STATUS_UNAVAIL
                   errno=97/EAFNOSUPPORT h_errno=-1/Resolver internal error

_nss_sss_gethostbyname2_r("fedoraproject.org", AF_UNIX) → status=NSS_STATUS_UNAVAIL
                   errno=97/EAFNOSUPPORT h_errno=-1/Resolver internal error

_nss_sss_gethostbyname_r("fedoraproject.org") → status=NSS_STATUS_NOTFOUND
                   errno=0/--- h_errno=-1/Resolver internal error