Test suite doesn't offer a way to override the default image

We currently offer [a configuration file](https://github.com/containers/toolbox/pull/828) to override the default image that's used for things like the `create` command. Some downstream distributors use this to significantly change the default image to the point that the test suite can't be run unless it uses the same default image that's specified in the configuration file.

As an aside, I am worried that the test suite seems to be growing it's own logic to resolve container and image names parallel to the actual Toolbox code itself. We should try to fix this, because the logic isn't trivial.

When running the test suite on RHEL, @olivergs was using a downstream patch to introduce some environment variables to override the defaults, but I don't see how it can possibly work with the state of the test suite in Toolbox 0.0.99.3.

I think that we need a proper upstream solution for this.

Review the configuration file introduced by pull request 828 and inspect how the test suite resolves container and image names. The work is complete when tests can use the configured default image, including downstream configurations, without requiring a downstream patch or separate environment-variable workaround.

Written by the indexing model from the issue text.