Improve errors and testing using NIOTS (#588)

Motivation

Currently error reporting with NIO Transport Services is often sub-par.
This occurs because the Network.framework connections may enter the
waiting state until the network connectivity state changes. We were not
watching for the user event that contains the error in that state, so if
we timed out in that state we'd just give a generic timeout error,
instead of telling the user anything more detailed.

Additionally, several of our tests assume that failure will be fast, but
in NIO Transport Services we will enter that .waiting state. This is
reasonable, as changed network connections may make a connection that
was not succeeding suddenly viable. However, it's inconvenient for
testing, where we're mostly interested in confirming that the error path
works as expected.

Modifications

- Add an observer of the WaitingForConnectivity event that records it
  into our state machine for later reporting.
- Add support for disabling waiting for connectivity for testing
  purposes.
- Add annotations to several tests to stop them waiting for
  connectivity.

Results

Faster tests, better coverage, better errors for our users.

Co-authored-by: David Nadoba <dnadoba@gmail.com>

Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift

@@ -406,6 +406,11 @@ extension HTTPConnectionPool {
             return .init(request: .none, connection: .scheduleBackoffTimer(connectionID, backoff: backoff, on: eventLoop))
         }
 
+        mutating func waitingForConnectivity(_ error: Error, connectionID: Connection.ID) -> Action {
+            self.lastConnectFailure = error
+            return .init(request: .none, connection: .none)
+        }
+
         mutating func connectionCreationBackoffDone(_ connectionID: Connection.ID) -> Action {
             // The naming of `failConnection` is a little confusing here. All it does is moving the
             // connection state from `.backingOff` to `.closed` here. It also returns the