Support guards in assert_push

[marcandre]

We can't use guards with assert_push and al., e.g.:

assert_push "presence_state", state when state == %{}

Environment

• Elixir version (elixir -v): Erlang/OTP 26 [erts-14.2.4] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]
• Phoenix version (mix deps): 1.7.12
• Operating system:

Actual behavior

    error: cannot invoke remote function :erlang.==/2 inside a match
    │
 49 │     assert_push "presence_state", state when state == %{}
    │                                                    ^
    │
    └─ test/enaia_web/channels/online_users_channel_test.exs:49:52: EnaiaWeb.OnlineUsersChannelTest.assert_state_push/1

Expected behavior

It would ideally produce code equivalent to

    assert_receive %Phoenix.Socket.Message{
                     event: "presence_state",
                     payload: state
                   }
                   when state == %{}

[josevalim]

A pull request will be welcome (please ping me if you send one!).