Add macaroon-specific tests

[aakselrod]

The macaroon code needs some tests! Some ideas for things to test include:

• Authenticating to one macaroon service with the macaroon of another service, with no macaroon, or with an invalid macaroon
• Authenticating using permissions that are denied or allowed in various ways
• Authenticating with macaroons containing expired and non-expired TimeBeforeCaveat
• Doing all of the above in both lnd integration tests (with and without lncli, thanks @mrwhythat) and package-level unit tests

[yashbhutwala]

Is anyone else working on this issue? If not, I can take a stab at it.

[merehap]

I'll take up the unit testing portion of this under the assumption that @yashbhutwala and @mrwhythat are no longer looking at this.