Mark test fields as readonly

[Kilazur]

Is your feature request related to a problem? Please describe.
When generating tests, I always have to mark all my fields as readonly manually.

Describe the solution you'd like
I believe test fields (for mocks usually) should be readonly by default, as their references are not meant to change during the execution of tests.

[mattwhitfield]

This is included in 0.210 👍