cmd/vet: let TestMain run deferred functions

Split TestMain into two functions so that we can defer cleanups.

Updates #30500

Change-Id: I1fa7957be0779c079ec4d221a8321b45ddb973e2
Reviewed-on: https://go-review.googlesource.com/c/164860
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/cmd/vet/vet_test.go

@@ -28,16 +28,19 @@ var binary string
 
 // We implement TestMain so remove the test binary when all is done.
 func TestMain(m *testing.M) {
+	os.Exit(testMain(m))
+}
+
+func testMain(m *testing.M) int {
 	dir, err := ioutil.TempDir("", "vet_test")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
-		os.Exit(1)
+		return 1
 	}
 	defer os.RemoveAll(dir)
 	binary = filepath.Join(dir, "testvet.exe")
 
-	result := m.Run()
-	os.Exit(result)
+	return m.Run()
 }
 
 var (