integration/client: Rework withBytesBuffers

All of the tests using this didn't need stdin/err (one of them not even
stdout), so we can just leave them "empty" and change to a withStdout
naming to make it more obvious.

Signed-off-by: Danny Canter <danny@dcantah.dev>

integration/client/container_checkpoint_test.go

@@ -488,7 +488,7 @@ func TestCheckpointRestoreWithImagePath(t *testing.T) {
 
 	stdout := bytes.NewBuffer(nil)
 	spec.Process.Args = []string{"ps", "-ef"}
-	process, err := ntask.Exec(ctx, t.Name()+"_exec", spec.Process, cio.NewCreator(withByteBuffers(stdout)))
+	process, err := ntask.Exec(ctx, t.Name()+"_exec", spec.Process, cio.NewCreator(withStdout(stdout)))
 	if err != nil {
 		t.Fatal(err)
 	}