Make simapp a standalone go module

- [x] Add `go.mod` to `simapp/`
- [x] Create CI assertion to maintain no dependencies on `simapp`
- [x] Add `go.mod` to `tests/`
- [x] Remove usages of tests/mocks from types/handler_test.go


This will close out Phase 3 of https://github.com/cosmos/cosmos-sdk/issues/11899