net: deflake TestPacketConn, increase timeout

net: deflake TestPacketConn, increase timeout

Tests sometimes failed with:

ok   mime/internal/quotedprintable      0.606s
ok   mime/multipart                     0.819s
--- FAIL: TestPacketConn (0.10s)
    packetconn_test.go:96: PacketConn.ReadFrom failed: WSARecvFrom udp 127.0.0.1:64156: i/o timeout
FAIL
FAIL    net     3.602s
ok      net/http        4.618s
ok      net/http/cgi    0.576s

Theory: 100 ms is too short. Small timer granularity on Wnidows, or an
allocation in the middle causing a GC sometimes?

In any case, increase it to 500 ms.

Change-Id: I48cc4d600eb168db9f85c0fd05335dd630254c3c
Reviewed-on: https://go-review.googlesource.com/4922
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>