Add test authenticator to make testing easy without any specific hardware

[Fraetor]

Issue #8 describes implementing a platform authenticator, detailing a bunch of security requirements and whatnot. While that will be desirable eventually, it would be good to have a platform-style authenticator available for testing with in the short term, until such an authenticator (or others, such as in #26) can be implemented.

It is important to avoid a test authenticator being used unwittingly when a proper authenticator should be, so it should probably have a couple properties:

1. Optionally compiled, so it isn't present in non-development builds.
2. Only store credentials in memory. By not persisting anything, there will be no temptation to try and use it for actual use.
3. Maybe restrict to a set list of origins, such as webauthn.io and other testing sites.

Adding such an authenticator would aid development by making it possible to test credentialsd without any specific hardware. Furthermore, it may also serve as a good test case for the API in #8.