cmd/stringer, go/loader: use testenv.NeedsTool(t, "cgo")

Using go env CGO_ENABLED has an advantage over build.Default.CgoEnabled
in that it pays attention to whether a C compiler is available in $PATH,
so it correctly skips the test when cgo is not enabled in more contexts.
I haven't found better docs for this than https://go.dev/doc/go1.20#cgo.

Also add this in TestCgoCwdIssue46877, since it requires cgo.

Change-Id: Ib14a983cc179b725a98f154846662887a91edc19
Reviewed-on: https://go-review.googlesource.com/c/tools/+/560175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

cmd/stringer/endtoend_test.go

@@ -13,7 +13,6 @@ import (
 	"bytes"
 	"flag"
 	"fmt"
-	"go/build"
 	"io"
 	"os"
 	"path"
@@ -50,6 +49,8 @@ func TestMain(m *testing.M) {
 }
 
 func TestEndToEnd(t *testing.T) {
+	testenv.NeedsTool(t, "go")
+
 	stringer := stringerPath(t)
 	// Read the testdata directory.
 	fd, err := os.Open("testdata")
@@ -76,8 +77,8 @@ func TestEndToEnd(t *testing.T) {
 			continue
 		}
 		t.Run(name, func(t *testing.T) {
-			if name == "cgo.go" && !build.Default.CgoEnabled {
-				t.Skipf("cgo is not enabled for %s", name)
+			if name == "cgo.go" {
+				testenv.NeedsTool(t, "cgo")
 			}
 			stringerCompileAndRun(t, t.TempDir(), stringer, typeName(name), name)
 		})
@@ -155,6 +156,8 @@ func TestTags(t *testing.T) {
 // TestConstValueChange verifies that if a constant value changes and
 // the stringer code is not regenerated, we'll get a compiler error.
 func TestConstValueChange(t *testing.T) {
+	testenv.NeedsTool(t, "go")
+
 	stringer := stringerPath(t)
 	dir := t.TempDir()
 	source := filepath.Join(dir, "day.go")

go/loader/loader_test.go

@@ -837,6 +837,7 @@ func loadIO(t *testing.T) {
 
 func TestCgoCwdIssue46877(t *testing.T) {
 	testenv.NeedsTool(t, "go")
+	testenv.NeedsTool(t, "cgo")
 	var conf loader.Config
 	conf.Import("golang.org/x/tools/go/loader/testdata/issue46877")
 	if _, err := conf.Load(); err != nil {

go/loader/stdlib_test.go

@@ -130,13 +130,11 @@ func TestCgoOption(t *testing.T) {
 	case "darwin":
 		t.Skipf("golang/go#58493: file locations in this test are stale on darwin")
 	}
+	testenv.NeedsTool(t, "go")
 	// In nocgo builds (e.g. linux-amd64-nocgo),
 	// there is no "runtime/cgo" package,
 	// so cgo-generated Go files will have a failing import.
-	if !build.Default.CgoEnabled {
-		return
-	}
-	testenv.NeedsTool(t, "go")
+	testenv.NeedsTool(t, "cgo")
 
 	// Test that we can load cgo-using packages with
 	// CGO_ENABLED=[01], which causes go/build to select pure