more info:
The steps to create a set of test tiers are identical to the steps you followed to create the WebLogic Administration Server and WebLogic Managed Server tiers. I've linked them here for your convenience. After you activate the tiers, note the IP addresses of the nodes in both test tiers. Use the node in the test WebLogic Administration Server tier as your image host.
Testing is most comprehensive when you use the same number of Managed Servers in your test tier as you specified for your service tier. If sufficient nodes are not available, you can test with fewer; be aware that testing may not reveal configuration errors with Managed Servers and machines beyond those you test.