clarification required on expected sign response in u2f tests

[apowers313]

Duplicating google/u2f-ref-code#138 . In the future it would be better to have these conversations in this repo.

[yackermann]

@apowers313 it seems that specs did not defined what are "message:error:test-of-user-presence-required" and "message:success" are.

Additionally the tool does not actually check correct error code return:

 // Sign with check only should not produce signature.
  PASS(test_Sign(0x6985, true));

uint32_t test_Sign(int expectedSW12 = 0x9000, bool checkOnly = false) {
...
  if (expectedSW12 != 0x9000) {
    CHECK_EQ(true, rsp.empty());
    return 0;
  }
...
}