fix silent failure in assert_dns_record test

If the name cannot be resolved, getent fails, which in turn causes
assert_dns_record to just return with a non-zero status at that point,
which causes the whole test to just terminate with a non-zero status,
but no indication whatsoever what had gone wrong.

test/config.sh

@@ -124,8 +124,7 @@ assert_dns_record() {
     exp_ips_regex=$(echo "$@" | sed -r 's/ /\\\|/g')
 
     [ -z "$DEBUG" ] || greyly echo "Checking whether $name exists at $host:$container"
-    got_ip=$(exec_on $host $container getent hosts $name)
-    assert_raises "echo '$got_ip' | grep -q '$exp_ips_regex'"
+    assert_raises "exec_on $host $container getent hosts $name | grep -q '$exp_ips_regex'"
 
     [ -z "$DEBUG" ] || greyly echo "Checking whether the IPs '$@' exists at $host:$container"
     for ip in "$@" ; do