manager/tests: validate JSON output from "kresctl cache clear --json" command

Author: alesmrazek

manager/tests/packaging/interactive/cache-clear.sh

@@ -7,9 +7,9 @@ if [ "$?" -ne "0" ]; then
 	exit 1
 fi
 
-# clear just example.com. AAAA record
-kresctl cache clear --exact-name --rr-type AAAA example.com.
+# clear just example.com. AAAA record, get JSON output
+kresctl cache clear --json --exact-name --rr-type AAAA example.com. | python3 -m json.tool
 if [ "$?" -ne "0" ]; then
-	echo "Could not clear example.com. AAAA record"
+	echo "Could not clear example.com. AAAA record or output is not a valid JSON"
 	exit 1
 fi