testing: add more parallelism to packetimpact dgram tests

This lowered runtime down to about [2
minutes](https://buildkite.com/gvisor/pipeline/builds/29983#018fca80-2991-4414-aa11-91c71874ea3b).

PiperOrigin-RevId: 639204164

test/packetimpact/tests/generic_dgram_socket_send_recv.go

@@ -86,6 +86,7 @@ func runAllCombinations(t *testing.T, proto protocolTest) {
 		dut.Net.RemoteIPv6,
 	} {
 		t.Run(fmt.Sprintf("bindTo=%s", bindTo), func(t *testing.T) {
+			t.Parallel()
 			for _, sendTo := range []net.IP{
 				net.IPv4bcast,
 				net.IPv4allsys,