Hi RicohN,
You won't be able to accomplish a fully separate network that can communicate outside of vSphere without doing some configuration on your network infrastructure (switches). What kind of switch(es) are you utilizing? I would say you'd want to create a new VLAN specifically for this test environment and place the VMNIC you're using with your vSwitch into this VLAN as an access port on the switch. After that, you can look at creating the AD/DCHP server. I would create an Interface VLAN and give it a subnet to use in your environment. You can then use that for your test subnet's gateway.