misc/cgo/stdio: fix tests in module mode

Updates #30228

Change-Id: I4d213c6fe68c47ccb877f13b55128e035f76a26b
Reviewed-on: https://go-review.googlesource.com/c/163421
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/stdio/stdio_test.go

@@ -0,0 +1,60 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package stdio_test
+
+import (
+	"bytes"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	log.SetFlags(log.Lshortfile)
+	os.Exit(testMain(m))
+}
+
+func testMain(m *testing.M) int {
+	GOPATH, err := ioutil.TempDir("", "cgostdio")
+	if err != nil {
+		log.Panic(err)
+	}
+	defer os.RemoveAll(GOPATH)
+	os.Setenv("GOPATH", GOPATH)
+
+	// Copy testdata into GOPATH/src/cgostdio, along with a go.mod file
+	// declaring the same path.
+	modRoot := filepath.Join(GOPATH, "src", "cgostdio")
+	if err := overlayDir(modRoot, "testdata"); err != nil {
+		log.Panic(err)
+	}
+	if err := os.Chdir(modRoot); err != nil {
+		log.Panic(err)
+	}
+	if err := ioutil.WriteFile("go.mod", []byte("module cgostdio\n"), 0666); err != nil {
+		log.Panic(err)
+	}
+
+	return m.Run()
+}
+
+func TestTestRun(t *testing.T) {
+	out, err := exec.Command("go", "env", "GOROOT").Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+	GOROOT := string(bytes.TrimSpace(out))
+
+	cmd := exec.Command("go", "run", filepath.Join(GOROOT, "test", "run.go"), "-", ".")
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("%s: %s\n%s", strings.Join(cmd.Args, " "), err, out)
+	}
+	t.Logf("%s:\n%s", strings.Join(cmd.Args, " "), out)
+}