Fix broken async tests.

lib/pure/asynchttpserver.nim

@@ -213,5 +213,5 @@ when isMainModule:
         "Content-type": "text/plain; charset=utf-8"}
     await req.respond(Http200, "Hello World", headers.newStringTable())
 
-  server.serve(TPort(5555), cb)
+  asyncCheck server.serve(TPort(5555), cb)
   runForever()

lib/pure/asyncnet.nim

@@ -271,7 +271,7 @@ when isMainModule:
           break
         else:
           echo("Got line: ", line)
-    main()
+    asyncCheck main()
   elif test == LowClient:
     var sock = newAsyncSocket()
     var f = connect(sock, "irc.freenode.net", TPort(6667))