remove Buffer from Mock*Service

[Geal]

Fix #1435

Note: this does not fix this issue yet, because the service is isolated in a task, so any panic generated by mockall will stay in the task: it is fixed now:

router/apollo-router/src/plugin/test/service.rs

Lines 26 to 35 in b55a628

	tokio::spawn(async move {
	loop {
	while let Some((request, responder)) = handle.next_request().await {
	match self.call(request) {
	Ok(response) => responder.send_response(response),
	Err(err) => responder.send_error(err),
	}
	}
	}
	});

[BrynCooke]

Nice!

[o0Ignition0o]

LGTM overall,

The only thing i m not comfortable with is the need for our plugin devs to call .expect_clone().

I think I'd even rather implement copy to our mocks to avoid having to do that.

WDYT is there a way we can get rid of this call?

[Geal]

it's not nice but AFAIK there is no way to just clone the expectations. And we can have different requirements depending on which instance of the service is called. I'd be open to a fix for that, but not in this PR: here I mainly want to have tests that work as expected