Well, actually, attempting to activate the same code on a second PAS will fail.
There are two ways you can approach this objective:
If the dev/test environment is network-connected to the production environment (well, even if it isn't, it's just tricker), you can install a Secondary Application Server (SAS), which will share the licensing from your existing Primary Application Server (PAS). You can then configure the SAS to be an identical clone of the PAS (in fact, most of the configuration, except user profiles, security roles, credentials and credential rings) will automatically replicate from the PAS.
For more information see this article:
Patch Manager Architecture – Deploying Application & Management Role Servers
If the dev/test network is isolated, you can install/configure the SAS from the connected network and then physically reconnect the SAS to the isolated network. As long as there are no major changes to the production network, the SAS will run just find in a disconnected environment.
For more information, see this article:
Using Patch Manager in a Disconnected Network Environment
Another alternative to the dev/test environment is to purchase a license of DameWare Patch Manager (50-node) for $495 -- but only in the most extreme and unfavorable conditions would this actually be necessary.