The best way is to create the test harness directly on the library block itself, not on each individual subsystem instance in your main model.
Here’s how you can do it:
1. Open your custom library.
2. Right-click the block you want to test.
3. Select Test Harness > Create for 'BlockName'.
Now, this harness will test the block’s behavior, and it applies to all instances of that block in your models. This way, you only need to maintain one test harness, and it covers every subsystem linked to that library block.
Hope this helps!
