Add trait to mock client dependency in DelayForwarder

[octol]

• Add trait for mocking the mixnode client dependency in the DelayForwarder.
• Create basic test instantiating the DelayForwarder and pass it a packet.

A alternative approach here could be to move the entire Client behind a trait instead of strictly what's needed for testing.

[jstuczyn]

Conceptually looks good to me. The only thing I'm maybe uncertain of is the name of the trait, i.e. SendWithoutResponse. I've been kinda liberally using send, send_with_response and send_without_response in multiple places, so I'm not sure if it might not be confusing later on. But I guess that's the problem for the future once other parts of the codebase are looked at.