TestKit: specialized fishForMessage, eventuallyExpectMessage(equalTo:)

[ktoso]

It would simplify code quite a bit sometimes, so we can avoid writing:

            try self.testKit(first).eventually(within: .seconds(3)) {
                guard try p1e.maybeExpectMessage()?.refs == [ref] else {
                    throw TestError("Expected \(ref) to be in the listing")
                }
            }

It could likely be best implemented using fishForMessage

[ktoso]

Maybe it should actually be

probe.fishForMessage(equalTo: ..., within: ...)

[ktoso]

Ignore that, it's expressible already as:

            try p1f.fishForMessages(within: .seconds(3)) {
                if $0.refs == [ref] { return .catchComplete
                } else { return .ignore }
            }