test: Use our own generate_cert helper

The current generate_cert helper was originally taken from Go's source,
and is more complex than we need it to be.

This patch replaces it with our own version, rewritten from scratch
independently.

test/util/lib.sh

@@ -186,7 +186,7 @@ function generate_certs_for() {
 	mkdir -p ${CONFDIR}/certs/${1}/
 	(
 		cd ${CONFDIR}/certs/${1}
-		generate_cert -ca -duration=1h -host=${1}
+		generate_cert -ca -validfor=1h -host=${1}
 	)
 }