cmd/go/internal/test: refresh flagdefs.go and fix test

The tests for cmd/go/internal/test were not running at all due to a
missed call to m.Run in TestMain. That masked two missing vet
analyzers ("directive" and "timeformat") and a missed update to the
generator script in CL 355452.

Fixes golang#58415.

Change-Id: I7b0315952967ca07a866cdaa5903478b2873eb7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/466635
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/test/flagdefs_test.go

@@ -9,13 +9,15 @@ import (
 	"cmd/go/internal/test/internal/genflags"
 	"flag"
 	"internal/testenv"
+	"os"
 	"reflect"
 	"strings"
 	"testing"
 )
 
 func TestMain(m *testing.M) {
 	cfg.SetGOROOT(testenv.GOROOT(nil), false)
+	os.Exit(m.Run())
 }
 
 func TestPassFlagToTestIncludesAllTestFlags(t *testing.T) {

src/cmd/go/internal/test/genflags.go

@@ -75,7 +75,7 @@ func testFlags() []string {
 		}
 
 		switch name {
-		case "testlogfile", "paniconexit0", "fuzzcachedir", "fuzzworker":
+		case "testlogfile", "paniconexit0", "fuzzcachedir", "fuzzworker", "gocoverdir":
 			// These flags are only for use by cmd/go.
 		default:
 			names = append(names, name)