TestLinksEtcHostsContentMatch: use container.Exec()

I am not quite sure why but this test is sometimes failing like this:

> 15:21:41 --- FAIL: TestLinksEtcHostsContentMatch (0.53s)
> 15:21:41 	assertions.go:226:
>
> 	Error Trace:	links_linux_test.go:46
> 15:21:41
> 	Error:      	Not equal:
> 15:21:41
> 	            	expected: "127.0.0.1\tlocalhost\n::1\tlocalhost
> ip6-localhost
> ip6-loopback\nfe00::0\tip6-localnet\nff00::0\tip6-mcastprefix\nff02::1\tip6-allnodes\nff02::2\tip6-allrouters\n172.17.0.2\tf53feb6df161\n"
> 15:21:41
> 	            	received: ""

To eliminate some possible failures (like ignoring stderr from `cat` or
its exit code), let's use container.Exec() to read a file from a container.

Fixes: e6bd20edcbf ("Migrate some integration-cli test to api tests")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

integration/container/links_linux_test.go

@@ -1,19 +1,15 @@
 package container // import "github.com/docker/docker/integration/container"
 
 import (
-	"bytes"
 	"context"
 	"io/ioutil"
 	"os"
 	"testing"
-	"time"
 
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/filters"
 	"github.com/docker/docker/integration/internal/container"
 	"github.com/docker/docker/integration/internal/request"
-	"github.com/docker/docker/pkg/stdcopy"
-	"github.com/gotestyourself/gotestyourself/poll"
 	"github.com/gotestyourself/gotestyourself/skip"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -29,21 +25,13 @@ func TestLinksEtcHostsContentMatch(t *testing.T) {
 	client := request.NewAPIClient(t)
 	ctx := context.Background()
 
-	cID := container.Run(t, ctx, client, container.WithCmd("cat", "/etc/hosts"), container.WithNetworkMode("host"))
-
-	poll.WaitOn(t, container.IsStopped(ctx, client, cID), poll.WithDelay(100*time.Millisecond))
-
-	body, err := client.ContainerLogs(ctx, cID, types.ContainerLogsOptions{
-		ShowStdout: true,
-	})
-	require.NoError(t, err)
-	defer body.Close()
-
-	var b bytes.Buffer
-	_, err = stdcopy.StdCopy(&b, ioutil.Discard, body)
+	cID := container.Run(t, ctx, client, container.WithNetworkMode("host"))
+	res, err := container.Exec(ctx, client, cID, []string{"cat", "/etc/hosts"})
 	require.NoError(t, err)
+	require.Empty(t, res.Stderr())
+	require.Equal(t, 0, res.ExitCode)
 
-	assert.Equal(t, string(hosts), b.String())
+	assert.Equal(t, string(hosts), res.Stdout())
 }
 
 func TestLinksContainerNames(t *testing.T) {