Add test for calling non-fully-active .store() / add automation?

[marcoscaceres]

Web Authn currently has Web Driver means to add a credential, which we could potentially leverage to test this... however, it would put a reliance on the Web Authn spec.

We should maybe consider a simple TestCredential that inherits from BasicCredential (only available "in automation") that we can use for testing Cred Man... thought that would require a virtual "test credentials" store, etc. That might be ok though if it seems worth the effort.