A client that can be useful for testing applications using Sleipnir. Can be used instead of a mock client, or a mock server.

Returns the push request passed to push to the pid specified when constructing the client in the form of {:push, request}.

delay_ms is an optional field that can be used to make the client "slow" - it will still send the message right at invocation, but only returns after the specified delay has elapsed. Defaults to 0.