object_store: add tests for OAuth emulation

ported from https://github.com/influxdata/object_store_rs/issues/46 by @crepererum

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
https://github.com/influxdata/object_store_rs/pull/45 left out the oauth2 part. We should set up some emulated oauth2 endpoint to actually test these code paths as well.

Start by reading the referenced object_store_rs issue 46 and pull request 45, then trace the existing OAuth2 code paths and test setup in this repository. Identify how an emulated OAuth2 endpoint can exercise those paths; done means the omitted OAuth2 behavior is covered by tests.

Written by the indexing model from the issue text.