Allow renewal of staging certs and honor --staging/--test flags.

It is nice to be able to reliably test cert renewals, such as when a cert is
requested in an application using a different method than it is being renewed
as, such as when a cert is obtained through standalone mode, and then expected
to renew through nginx/apache.

Testing renewals by running the cron command swaps out staging cert requests
for production ones.

Operators may want to test the renewal command and append --force --test
of the form:
`acme.sh --cron --home="/home/letsencrypt" --force --test`

make the --test flag (or --staging) flag on this command honor the request to
continue to use staging servers for renewals when requested.

related to #6368

acme.sh

@@ -5454,6 +5454,7 @@ renew() {
   . "$DOMAIN_CONF"
   _debug Le_API "$Le_API"
 
+  if [ -z "$STAGE" ]; then
     case "$Le_API" in
     "$CA_LETSENCRYPT_V2_TEST")
       _info "Switching back to $CA_LETSENCRYPT_V2"
@@ -5468,6 +5469,7 @@ renew() {
       Le_API="$CA_GOOGLE"
       ;;
     esac
+  fi
 
   if [ "$_server" ]; then
     Le_API="$_server"