tests/int: show stderr if command failed part II

This adds a few cases missed by commit bf4fcc30.

Fixes: bf4fcc30
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/exec_test.go

@@ -784,6 +784,7 @@ func TestPassExtraFiles(t *testing.T) {
 		ExtraFiles: []*os.File{pipein1, pipein2},
 		Stdin:      nil,
 		Stdout:     &stdout,
+		Stderr:     new(strings.Builder),
 		Init:       true,
 	}
 	err = container.Run(&process)

libcontainer/integration/execin_test.go

@@ -421,6 +421,7 @@ func TestExecinPassExtraFiles(t *testing.T) {
 		ExtraFiles: []*os.File{pipein1, pipein2},
 		Stdin:      nil,
 		Stdout:     &stdout,
+		Stderr:     new(strings.Builder),
 	}
 	err = container.Run(inprocess)
 	ok(t, err)