server_test: wait for OnClose in TestClientEOF.

In the test for client Call failing with ErrClosed on a closed
server, wait for the client's OnClose handler to get triggered
to make sure closing the socket had properly been administered
on the client's side. Otherwise trying a new Call() might fail
with some other error than ErrClosed, for instance ENOTCONN.

Signed-off-by: Krisztian Litkey <krisztian.litkey@intel.com>

server_test.go

@@ -363,6 +363,8 @@ func TestClientEOF(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	client.UserOnCloseWait(ctx)
+
 	// server shutdown, but we still make a call.
 	if err := client.Call(ctx, serviceName, "Test", tp, tp); err == nil {
 		t.Fatalf("expected error when calling against shutdown server")