net: unskip TestUDPZeroBytePayload on Darwin

From information we have available in the issue, this test was flaky
on OS X 10.10 but not newer. By now macOS 11 is the minimum required
version, and 1000 local runs of the test passed, so try to unskip as
the next step.

For #29225.

Change-Id: I18a24459f01e53ef9de05b50d1dd8786d0683107
Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64_11,gotip-darwin-amd64_14
Reviewed-on: https://go-review.googlesource.com/c/go/+/601395
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: dmitshur

src/net/udpsock_test.go

@@ -340,7 +340,7 @@ func TestUDPZeroBytePayload(t *testing.T) {
 	switch runtime.GOOS {
 	case "plan9":
 		t.Skipf("not supported on %s", runtime.GOOS)
-	case "darwin", "ios":
+	case "ios":
 		testenv.SkipFlaky(t, 29225)
 	}
 	if !testableNetwork("udp") {