Create a self-test command

[ewjoachim]

It would do a set, a list and get, and finally a delete and return with exit code zero if it all went well

[ewjoachim]

actually, we can introspect the current token, this might be easier to do.
(well, nothing beats a real test but...)

[ewjoachim]

There's now vault lookup-token, we could add a --policy that would also fetch policy definitions.