internal/synctest: speed up TestWeak

Run TestWeak for fewer iterations. Five is enough reproduce #73817,
which was the motivation for this test. runtime.GC is ridiculously
slow on wasm, and not especially fast anywhere else.

Change-Id: Ieb5235d064b123cbc22a306425e385c273b54493
Reviewed-on: https://go-review.googlesource.com/c/go/+/675716
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/synctest/synctest_test.go

@@ -710,7 +710,7 @@ func TestHappensBefore(t *testing.T) {
 // https://go.dev/issue/73817
 func TestWeak(t *testing.T) {
 	synctest.Run(func() {
-		for range 100 {
+		for range 5 {
 			runtime.GC()
 			b := make([]byte, 1024)
 			weak.Make(&b)