bpf, testing: Add missing object file to TEST_FILES

When installing kselftests to its own directory and run the
test_lwt_ip_encap.sh it will complain that test_lwt_ip_encap.o can't be
found. Same with the test_tc_edt.sh test it will complain that
test_tc_edt.o can't be found.

  $ ./test_lwt_ip_encap.sh
  starting egress IPv4 encap test
  Error opening object test_lwt_ip_encap.o: No such file or directory
  Object hashing failed!
  Cannot initialize ELF context!
  Failed to parse eBPF program: Invalid argument

Rework to add test_lwt_ip_encap.o and test_tc_edt.o to TEST_FILES so the
object file gets installed when installing kselftest.

Fixes: 74b5a596

 ("selftests/bpf: Replace test_progs and test_maps w/ general rule")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191111161728.8854-1-anders.roxell@linaro.org