You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: server_rtt_test.go
    path of file: ./repos/learning/golang/practice/go-crontab/gopath/src/github.com/mongodb/mongo-go-driver/core/topology
    
    the code of the file until where you have to start completion: // Copyright (C) MongoDB, Inc. 2017-present.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

package topology

import (
	"encoding/json"
	"io/ioutil"
	"path"
	"testing"
	"time"

	"github.com/mongodb/mongo-go-driver/internal/testutil/helpers"
	"github.com/stretchr/testify/require"
)

// Test case for all server selecti
    
func TestServerSelectionRTTSpec(t *testing.T) {

	type testCase struct {
		AvgRttMs  json.Number `json:"avg_rtt_ms"`
		NewRttMs  float64     `json:"new_rtt_ms"`
		NewAvgRtt float64     `json:"new_avg_rtt"`
	}

	const testsDir string = "../../data/server-selection/rtt"

	for _, file := range testhelpers.FindJSONFilesInDir(t, testsDir) {
		func(t *testing.T, filename string) {
			filepath := path.Join(testsDir, filename)
			content, err := ioutil.ReadFile(filepath)
			require.NoError(t, err)

			// Remove ".json" from filename.
			testName := filename[:len(filename)-5]

			t.Run(testName, func(t *testing.T) {
				var test testCase
				require.NoError(t, json.Unmarshal(content, &test))

				var server Server

				if test.AvgRttMs != "NULL" {
					avg, err := test.AvgRttMs.Float64()
					require.NoError(t, err)

					server.averageRTT = time.Duration(avg * float64(time.Millisecond))
					server.averageRTTSet = true
				}

				server.updateAverageRTT(time.Duration(test.NewRttMs * float64(time.Millisecond)))
				require.Equal(t, server.averageRTT, time.Duration(test.NewAvgRtt*float64(time.Millisecond)))
			})
		}(t, file)
	}
}