time: skip test that will fail with GO111MODULE=off

The test is designed to ensure that behavior introduced in Go 1.23
to garbage collect async timed channels is working correctly. If
GO111MODULE=off is set (or GODEBUG=asynctimerchan=1) Go reverts to the
Go 1.20 behavior of not garbage collecting these channels, which fails
the test.

Instead of running a test in conditions where we know it will fail,
just skip the test. A more comprehensive test does not make sense
right now because this code may go away soon.

Fixes #76948.

Change-Id: Ib186abd2ea583a06b5c246bfd6df932522cf7f1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/732100
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/time/export_test.go

@@ -40,6 +40,7 @@ var (
 	Tzset                  = tzset
 	TzsetName              = tzsetName
 	TzsetOffset            = tzsetOffset
+	AsynctimerChan         = asynctimerchan
 )
 
 func LoadFromEmbeddedTZData(zone string) (string, error) {

src/time/tick_test.go

@@ -266,6 +266,10 @@ func BenchmarkTickerResetNaive(b *testing.B) {
 }
 
 func TestTimerGC(t *testing.T) {
+	if AsynctimerChan.Value() == "1" {
+		t.Skip("skipping TestTimerGC with asynctimerchan=1")
+	}
+
 	run := func(t *testing.T, what string, f func()) {
 		t.Helper()
 		t.Run(what, func(t *testing.T) {