Trouble with Mox when testing a GenServer calling a mock in `init/1`

I recently ran into a tricky error while testing a GenServer that uses a mock in its init/1.
Maybe this helps others, or maybe someone has cleaner ideas.

The error looked like this:

** (Mox.UnexpectedCallError) no expectation defined for Supex.Sclang.ScPortMock.open/2
in process #PID<0.206.0> with args [{:spawn, "sclang"}, [:binary]]

At first, I tried the usual:

    test "Sclang GenServer state ok" do
      Supex.Sclang.ScPortMock
      |> expect(:open, fn _name, _opts -> Port.open({:spawn, "cat"}, [:binary]) end)

      assert {:ok, _pid} = start_supervised(Sclang)
      assert %Sclang{} = :sys.get_state(Sclang)
    end

But that failed because the GenServer calls the mock inside init/1.
By the time init/1 runs, the supervised process is in a different pid,
so the expectation defined in the test process didn’t apply.

The missing piece was using Mox.allow/3 with a function that resolves the pid later:

    test "Sclang GenServer state ok" do
      Supex.Sclang.ScPortMock
      |> expect(:open, fn _name, _opts -> Port.open({:spawn, "cat"}, [:binary]) end)

      Supex.Sclang.ScPortMock
      |> allow(self(), fn -> GenServer.whereis(Sclang) end)

      assert {:ok, _pid} = start_supervised(Sclang)
      assert %Sclang{} = :sys.get_state(Sclang)
    end

The key is that allow/3 accepts a function, not just a pid.
When the GenServer invokes the mock in init/1, Mox evaluates the function,
resolves the pid, and lets the call go through.

This finally got rid of the Mox.UnexpectedCallError.