s3:tests: Fix shellcheck warning of test_wbinfo_lookuprids_cache.sh

s3:tests: Fix shellcheck warning of test_wbinfo_lookuprids_cache.sh

In source3/script/tests/test_wbinfo_lookuprids_cache.sh line 27:
key=$("$TDBDUMP" "$cache" | grep ^key.*NDR.*/"$opnum"/ | cut -d\" -f2)
                                 ^-------------------^ SC2062 (warning):
                                 Quote the grep pattern so the shell won't
                                 interpret it.

Lets better use awk for matching the pattern.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>