selftests: bpf: test_lwt_ip_encap: add negative tests.

As requested by David Ahern:

- add negative tests (no routes, explicitly unreachable destinations)
  to exercize error handling code paths;
- do not exit on test failures, but instead print a summary of
  passed/failed tests at the end.

Future patches will add TSO and VRF tests.

Signed-off-by: Peter Oskolkov <posk@google.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>