save wait_for_message contents

(cherry picked from commit 1334daaec0)

bin/tests/system/catz/clean.sh

@@ -24,3 +24,4 @@ rm -f ns[123]/catalog[1234].example.db
 rm -rf ns2/zonedir
 rm -f ns*/*.nzd ns*/*.nzd-lock
 rm -f ns*/managed-keys.bind*
+rm -f wait_for_message.*

bin/tests/system/catz/tests.sh

@@ -24,7 +24,8 @@ rndccmd() (
 )
 
 _wait_for_message() (
-	nextpartpeek "$1" | grep -F "$2" >/dev/null
+	nextpartpeek "$1" > wait_for_message.$n
+	grep -F "$2" wait_for_message.$n >/dev/null
 )
 
 wait_for_message() (