oauth-client - Add test for existing Auth Code flow behavior

[highfalutin]

Preparation for work on the OAuth client extension to support OAuth tokens linked to particular contacts, as discussed with @totten at https://lab.civicrm.org/dev/core/-/issues/2181#note_58421

Before I make any PRs that change the inner workings of the auth code flow, this adds a test that asserts the existing behavior as a baseline.

[civibot]

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[eileenmcnaughton]

Looks like jenkins doesn't love this as much as I do https://test.civicrm.org/job/CiviCRM-Core-PR/40573/checkstyleResult/new/

[highfalutin]

oh Jenkins! PHPStorm is formatting my code to PSR-12 standards. Do you happen to know how I can tell it to use the style that Jenkins likes?

[highfalutin]

I think I wasn't understanding https://docs.civicrm.org/dev/en/latest/tools/phpstorm/#code-style-setup. I'll reformat.

[seamuslee001]

this looks fine to me @totten ?

[totten]

Ooh, yes, please. More test coverage. :)

This is a test-only enhancement, so there's no r-run/r-user/etc. Looks pretty good and passes tests. Merging.