[oci implementation] Test all authentication mechanisms

[joshspicer]

#460 is adding in support for docker credential helpers. There's existing test coverage on the "simpler" auth mechanisms but not on all.

This likely means utlizing runners for macOS, linux, and windows mocking out the credential helper api. This would also mean needing to embed credentials in the test runner to fetch private Features based on the credentials. (I personally use privatedevcontainercli.azurecr.io/features/a, but those secrets aren't available to CI yet).

cc/ @aaronlehmann