JwtBearerOptions.SaveToken tests

[Tratcher]

SaveToken does not have any tests and it regressed in 2.1. See #1767

@HaoK

[HaoK]

Are we sure this regressed in 2.1 as opposed to never working in 2.0? (given that we never had tests :)

[HaoK]

I didn't even realize save tokens was a thing for JwtBearer, I thought it was an OAuth only feature

[Tratcher]

FYI: GoogleTests covers this for the interactive OAuth flow. JwtBearer and OIDC are lacking test coverage though.
It's also covered in the sample apps use for manual testing.