plugin/forward: close channels when connManager returns (#1727)

Close a bunch of channels, also change the test to just use a for loop
with a counter.

plugin/forward/persistent.go

@@ -44,7 +44,14 @@ func newTransport(addr string, tlsConfig *tls.Config) *transport {
 		ret:    make(chan connErr),
 		stop:   make(chan bool),
 	}
-	go t.connManager()
+	go func() {
+		t.connManager()
+		// if connManager returns it has been stopped.
+		close(t.stop)
+		close(t.yield)
+		close(t.dial)
+		// close(t.ret) // we can still be dialing and wanting to send back the socket on t.ret
+	}()
 	return t
 }