Better test incoming connections.

Better test incoming connections.

0. Factor the TCP accept tests into their own test class.
1. Test all routing modes. Previously, the code was only testing
   incoming connections with SO_BINDTODEVICE and mark accepting.
   Test with UID and explicit marking as well.
2. Also check that when data is sent on the connection, it goes
   out on the right interface.
3. In between calls, attempt to clear the socket destination
   cache by changing IPv4 ToS and adding / deleting IPv6 options.

Change-Id: I15a28aa1f657f9c876e622e8a1a07185976828e3