Fix instanceof checks with Jest mocks

If you’re checking a variable type with instanceof and it fails with mocked objects, the prototype chain is broken. Fix it by constructing the mock properly:

someMock = Object.assign(
  Object.create(TheClass.prototype),
  { override: jest.fn() }
)

Alternatively:

Object.setPrototypeOf(someMock, TheClass.prototype)

This ensures someMock instanceof TheClass returns true.