Move `Default` instances to `testlib`

[lehins]

For the most part Default is used only for testing. In fact, it should only be used for testing. Therefore, we should move all Default instances into the testlib, similarly to how we provide instances for Arbitrary and ToExpr

[Soupstraw]

I tried to implement this in #5167, but I ran into issues with the default implementation of initialRules, because that depends on Default (State rule). @lehins thought it's better to solve this issue once we have integrated his new STS library into our codebase.