integrate ioutil migration in test

Some inconsistent change have been introduced in a previous version of ioutil migration(0c3105691f).

testing/scenarios/common.go

@@ -5,7 +5,7 @@ import (
 	"crypto/rand"
 	"fmt"
 	"io"
-	"io/ioutil"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"

testing/scenarios/feature_test.go

@@ -3,7 +3,7 @@ package scenarios
 import (
 	"context"
 	"google.golang.org/protobuf/types/known/anypb"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"net/url"
 	"testing"

testing/scenarios/http_test.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"crypto/rand"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 	"testing"