The ._anvil_designer module is generated at runtime in the browser to produce the FormTemplate class from the yaml.
You may want to check out @owen.campbell test library that may do what you need.
It doesn’t seem like it’s what you’re after, but you could also look at using the client side unittest module. I’m not sure how many have tried doing that. I haven’t seen many questions about using it so I assume not. But I’ve used it a fair bit so can point in the right direction if you go down that route.