Enable calling Mimic.allow before expectations have been defined

[michaelst]

Similar to Hammox it would be nice to allow processes to use a mock before expectations have been defined.

In this case I have a genserver that takes a opt for allow_fun which will run a provided function in the genserver process on init

def init(opts) do
  if fun = opts[:allow_fun], do: fun.(self())

  {:ok, []}
end

In the test I then pass the allow_fun

allow_fun = fn pid ->
  Ecto.Adapters.SQL.Sandbox.allow(Auth.Repo, self, pid)
  Mimic.allow(Repo, self, pid)
end

key_server = start_supervised!({Auth.Signing.Guardian.KeyServer, allow_fun: allow_fun})

However, with this setup I have to put the expects before starting the genserver, which prevents me from locating them next to the code that will trigger the expects.