cmd/internal/testdir: print stderr if go list fails while gathering stdlib import config

If cmd/compile is in an unhappy state, the testdir test can
fail with an unhelpful 'exit code 1' log message if
'go list' fails while gathering stdlib import config
When running individual files, such as:

 go test cmd/internal/testdir -run='Test/escape.*.go'

This might also happen in other uses, or it might be
that a more expansive set of tests such as run.bash
might first trigger a more useful error.

This change prints stderr and states that it is 'go list'
that is having problems to help someone track down the
proper issue.

Change-Id: Iba658ea139bb9087ab8adb00c9f65080a1b6ee76
Reviewed-on: https://go-review.googlesource.com/c/go/+/524941
Reviewed-by: Matthew Dempsky <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: thepudds

src/cmd/internal/testdir/testdir_test.go

@@ -215,8 +215,11 @@ var stdlibImportcfg = sync.OnceValue(func() string {
 	cmd := exec.Command(goTool, "list", "-export", "-f", "{{if .Export}}packagefile {{.ImportPath}}={{.Export}}{{end}}", "std")
 	cmd.Env = append(os.Environ(), "GOENV=off", "GOFLAGS=")
 	output, err := cmd.Output()
+	if err, ok := err.(*exec.ExitError); ok && len(err.Stderr) != 0 {
+		log.Fatalf("'go list' failed: %v: %s", err, err.Stderr)
+	}
 	if err != nil {
-		log.Fatal(err)
+		log.Fatalf("'go list' failed: %v", err)
 	}
 	return string(output)
 })