none/tests/res_search.vgtest: Add prereq that domain can be resolved.

Some autobuilders/testers don't allow network access.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14385 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/none/tests/res_search.vgtest b/none/tests/res_search.vgtest
index 879ce27..2253830 100644
--- a/none/tests/res_search.vgtest
+++ b/none/tests/res_search.vgtest
@@ -1,3 +1,4 @@
+prereq: host www.yahoo.com > /dev/null
 prog: res_search
 args: www.yahoo.com
 vgopts: -q