Use a better way to invalidate socket destination cache entries.

Some of the TCP tests rely on invalidating socket destination
cache entries to ensure that packets are routed correctly because
the routing code is correct, not because the destination cache
entry is correct. However, we don't currently have a way to
invalidate the destination cache entry of a socket that has no
corresponding userspace socket, so we can't check that
established but not yet accepted sockets are routed correctly.

Instead of invalidating socket destination caches by relying on
the side effects of changing IP options, manipulate routing to
the socket destination address. This works regardless of whether
we have a socket fd or not.

Also augment testIPv[v6]Remarking to test that InvalidateDstCache
actually does something.

Change-Id: I043b043699571b4553243bd10034d65e50003b1b