Remove `mutable=True` in test suite

[williambdean]

Description

There are few instances of mutable=True in the test suite which raise warnings while running.

Search on repo: https://github.com/search?q=repo%3Apymc-devs%2Fpymc%20mutable%3DTrue&type=code

Would likely be best to just remove this argument since they are not checked

[ricardoV94]

Thanks, do you want to do it?