net: add even more timing slop for TestDialParallel

This test expects dials of a closed port to complete in about the same
amount of time: an initial probe value +/- 20%. Reduce test flakes on
Windows by increasing the slop to +/- 50% of the original value.

Fixes #52173

Change-Id: I813492c36aca2b0264b3b5b8c96e8bf97193af76
Reviewed-on: https://go-review.googlesource.com/c/go/+/408354
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/dial_test.go

@@ -302,8 +302,8 @@ func TestDialParallel(t *testing.T) {
 		// We used to always use 95 milliseconds as the slop,
 		// but that was flaky on Windows.  See issue 35616.
 		slop := 95 * time.Millisecond
-		if fifth := tt.expectElapsed / 5; fifth > slop {
-			slop = fifth
+		if half := tt.expectElapsed / 2; half > slop {
+			slop = half
 		}
 		expectElapsedMin := tt.expectElapsed - slop
 		expectElapsedMax := tt.expectElapsed + slop