update go client test dependencies (#468)

samples/client/petstore/go/auth_test.go

@@ -1,14 +1,13 @@
 package main
 
 import (
+	"context"
 	"net/http"
 	"net/http/httputil"
 	"strings"
 	"testing"
 	"time"
 
-	"golang.org/x/net/context"
-
 	"golang.org/x/oauth2"
 
 	sw "./go-petstore"

samples/client/petstore/go/pet_api_test.go

@@ -1,15 +1,15 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"testing"
 
-	sw "./go-petstore"
-	"golang.org/x/net/context"
-
 	"github.com/antihax/optional"
 	"github.com/stretchr/testify/assert"
+
+	sw "./go-petstore"
 )
 
 var client *sw.APIClient

samples/client/petstore/go/store_api_test.go

@@ -1,11 +1,11 @@
 package main
 
 import (
+	"context"
 	"testing"
 	"time"
 
 	sw "./go-petstore"
-	"golang.org/x/net/context"
 )
 
 func TestPlaceOrder(t *testing.T) {

samples/client/petstore/go/user_api_test.go

@@ -1,12 +1,12 @@
 package main
 
 import (
+	"context"
 	"testing"
 
-	sw "./go-petstore"
-	"golang.org/x/net/context"
-
 	"github.com/stretchr/testify/assert"
+
+	sw "./go-petstore"
 )
 
 func TestCreateUser(t *testing.T) {