Make testIPv6PktinfoRouting pass on 32-bit Python.

On 32-bit Python interpreters, ntohl(0xbeef), i.e., 0xefbe0000,
has type long, and its type remains long even when cast to int.
This causes _MakeMsgControl to fail because we don't how to make
a cmsg with a long type. Fix this by using a flow label of 1234
instead.

Bug: 36510518
Test: testIPv6StickyPktinfo passes on device with 32-bit Python
Test: all_tests.sh passes on android-4.9
Change-Id: Ica311d44fe9687a3581e0f5013aa3cd2fa1f5862