sync: only check for successful PopHeads in long mode

In TestPoolDequeue it's surprisingly common for the queue to stay
nearly empty the whole time and for a racing PopTail to happen in the
window between the producer doing a PushHead and doing a PopHead. In
short mode, there are only 100 PopTail attempts. On linux/amd64, it's
not uncommon for this to fail 50% of the time. On linux/arm64, it's
not uncommon for this to fail 100% of the time, causing the test to
fail.

This CL fixes this by only checking for a successful PopTail in long
mode. Long mode makes 200,000 PopTail attempts, and has never been
observed to fail.

Fixes #31422.

Change-Id: If464d55eb94fcb0b8d78fbc441d35be9f056a290
Reviewed-on: https://go-review.googlesource.com/c/go/+/183981
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/sync/pool_test.go

@@ -236,10 +236,12 @@ func testPoolDequeue(t *testing.T, d PoolDequeue) {
 			t.Errorf("expected have[%d] = 1, got %d", i, count)
 		}
 	}
-	if nPopHead == 0 {
-		// In theory it's possible in a valid schedule for
-		// popHead to never succeed, but in practice it almost
-		// always succeeds, so this is unlikely to flake.
+	// Check that at least some PopHeads succeeded. We skip this
+	// check in short mode because it's common enough that the
+	// queue will stay nearly empty all the time and a PopTail
+	// will happen during the window between every PushHead and
+	// PopHead.
+	if !testing.Short() && nPopHead == 0 {
 		t.Errorf("popHead never succeeded")
 	}
 }