Make the SOCK_DESTROY tests work again.

These bitrotted due to changes to the tests developed on kernels
which don't yet have SOCK_DESTROY backported to them. Make minor
fixes to get them passing again:

1. Move asserts shared between TcpTest and SockDiagTest to
   SockDiagBaseTest.
2. Make IncomingConnection not leave FIN_WAIT sockets around,
   and check for their presence in testCrossFamilyBytecode, which
   fails if any other sockets exist.
3. Make mapped addresses work in testConnectInterrupted.

Change-Id: I035cd9b6570ca83a411d6e12df46ce3e76554a19