Add unit tests

[nsatragno]

Add tests, at the very least to cover the following scenarios:

• Constructing a webauthn object with nonsensical parameters.
• Generating a challenge.
• Creating a credential (response with an attestation).
  • Error cases (using the credential creation steps from the spec)
  • Correct case
  • With and without attestation
  • With and without UV
• Getting an assertion (response with the assertion).
  • Error cases (using the verifying an assertion steps from the spec)
  • Correct case
  • With and without UV
• login with matching credential and without matching credential.