go/importer,os/exec: use testenv.GoToolPath

These were the last two occurences of exec.Command("go", ...) in all of
std cmd. Checked with:

	gogrep '$(f is(func))("go", $*_)' std cmd

Also changed lp_windows_test to use a test package name to avoid a
circular dependency, since internal/testenv imports os/exec.

Change-Id: I9a18948600dfecc8507ad76172e219e78b791ffd
Reviewed-on: https://go-review.googlesource.com/87200
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/importer/importer_test.go

@@ -17,7 +17,7 @@ func TestFor(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 
 	const thePackage = "math/big"
-	out, err := exec.Command("go", "list", "-f={{context.Compiler}}:{{.Target}}", thePackage).CombinedOutput()
+	out, err := exec.Command(testenv.GoToolPath(t), "list", "-f={{context.Compiler}}:{{.Target}}", thePackage).CombinedOutput()
 	if err != nil {
 		t.Fatalf("go list %s: %v\n%s", thePackage, err, out)
 	}

src/os/exec/lp_windows_test.go

@@ -2,13 +2,18 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package exec
+// Use an external test to avoid os/exec -> internal/testenv -> os/exec
+// circular dependency.
+
+package exec_test
 
 import (
 	"fmt"
+	"internal/testenv"
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -63,7 +68,7 @@ type lookPathTest struct {
 }
 
 func (test lookPathTest) runProg(t *testing.T, env []string, args ...string) (string, error) {
-	cmd := Command(args[0], args[1:]...)
+	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Env = env
 	cmd.Dir = test.rootDir
 	args[0] = filepath.Base(args[0])
@@ -346,7 +351,7 @@ func (test commandTest) isSuccess(rootDir, output string, err error) error {
 }
 
 func (test commandTest) runOne(rootDir string, env []string, dir, arg0 string) error {
-	cmd := Command(os.Args[0], "-test.run=TestHelperProcess", "--", "exec", dir, arg0)
+	cmd := exec.Command(os.Args[0], "-test.run=TestHelperProcess", "--", "exec", dir, arg0)
 	cmd.Dir = rootDir
 	cmd.Env = env
 	output, err := cmd.CombinedOutput()
@@ -532,7 +537,7 @@ func buildPrintPathExe(t *testing.T, dir string) string {
 		t.Fatalf("failed to execute template: %v", err)
 	}
 	outname := name + ".exe"
-	cmd := Command("go", "build", "-o", outname, srcname)
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", outname, srcname)
 	cmd.Dir = dir
 	out, err := cmd.CombinedOutput()
 	if err != nil {