Add deny function for further Mox compatibility

Mox added a [deny/3](https://hexdocs.pm/mox/Mox.html#deny/3) function in [v1.2.0](https://hexdocs.pm/mox/Mox.html#deny/3). I was trying to test something by asserting a function hadn't been called and was routed to `deny/3` in Mox, only to find that it's not yet implemented in Hammox. Would be very nice to have here as well.