net: bump TestDialerDualStackFDLeak timeout on iOS

On an iPhone 6 running iOS 11, the TestDialerDualStackFDLeak test
started failing with dial durations just above the limit:

FAIL: TestDialerDualStackFDLeak (0.21s)

	dial_test.go:90: got 101.154ms; want <= 95ms

Bump the timeout on iOS.

For the iOS builder.

Change-Id: Id42b471e7cf7d0c84f6e83ed04b395fa1a2d449d
Reviewed-on: https://go-review.googlesource.com/66491
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dial_test.go

@@ -161,6 +161,8 @@ func dialClosedPort() (actual, expected time.Duration) {
 	// but other platforms should be instantaneous.
 	if runtime.GOOS == "windows" {
 		expected = 1500 * time.Millisecond
+	} else if runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64") {
+		expected = 150 * time.Millisecond
 	} else {
 		expected = 95 * time.Millisecond
 	}