selftests: bpf: Make reuseport test output more legible

Include the name of the mismatching result in human readable format
when reporting an error. The new output looks like the following:

  unexpected result
   result: [1, 0, 0, 0, 0, 0]
  expected: [0, 0, 0, 0, 0, 0]
  mismatch on DROP_ERR_INNER_MAP (bpf_prog_linum:153)
  check_results:FAIL:382

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200124112754.19664-4-lmb@cloudflare.com