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/tasyncsend4757.nim

@@ -1,24 +1,24 @@
-discard """
-output: "Finished"
-"""
-
 import asyncdispatch, asyncnet
 
-proc createServer(port: Port) {.async.} =
+var port: Port
+proc createServer() {.async.} =
   var server = newAsyncSocket()
   server.setSockOpt(OptReuseAddr, true)
-  bindAddr(server, port)
+  bindAddr(server)
+  port = getLocalAddr(server)[1]
   server.listen()
   while true:
     let client = await server.accept()
     discard await client.recvLine()
 
-asyncCheck createServer(10335.Port)
+asyncCheck createServer()
 
+var done = false
 proc f(): Future[void] {.async.} =
-  let s = newAsyncNativeSocket()
-  await s.connect("localhost", 10335.Port)
+  let s = createAsyncNativeSocket()
+  await s.connect("localhost", port)
   await s.send("123")
-  echo "Finished"
+  done = true
 
 waitFor f()
+doAssert done