fix #14320 (tasyncawait.nim is recently very flaky) + avoid hardcoding service ports everywhere + flakyAssert (#14327)

* hotfix #14320 tasyncawait.nim is recently very flaky
* fix #14327
* add flakyAssert

tests/async/tasyncawait.nim

@@ -1,8 +1,5 @@
-discard """
-  output: "2000"
-"""
-import asyncdispatch, asyncnet, nativesockets, net, strutils, os
-
+import asyncdispatch, asyncnet, nativesockets, net, strutils
+from stdtest/netutils import bindAvailablePort
 var msgCount = 0
 
 const
@@ -42,26 +39,18 @@ proc readMessages(client: AsyncFD) {.async.} =
       else:
         doAssert false
 
-proc createServer(port: Port) {.async.} =
-  var server = createAsyncNativeSocket()
-  block:
-    var name: Sockaddr_in
-    name.sin_family = typeof(name.sin_family)(toInt(AF_INET))
-    name.sin_port = htons(uint16(port))
-    name.sin_addr.s_addr = htonl(INADDR_ANY)
-    if bindAddr(server.SocketHandle, cast[ptr SockAddr](addr(name)),
-                sizeof(name).Socklen) < 0'i32:
-      raiseOSError(osLastError())
-
+proc createServer(server: AsyncFD) {.async.} =
   discard server.SocketHandle.listen()
   while true:
     asyncCheck readMessages(await accept(server))
 
-asyncCheck createServer(Port(10335))
-asyncCheck launchSwarm(Port(10335))
+let server = createAsyncNativeSocket()
+let port = bindAvailablePort(server.SocketHandle)
+asyncCheck createServer(server)
+asyncCheck launchSwarm(port)
 while true:
   poll()
   if clientCount == swarmSize: break
 
 assert msgCount == swarmSize * messagesToSend
-echo msgCount
+doAssert msgCount == 2000