Modelbench SUT cleanup + testing infra improvements

[bkorycki]

These are some initial improvements along the road to removing the SUT wrapper class in modelbench. I'm splitting the changes up into smaller PRs to make the more manageable to review.

Changes in this PR:

• Get rid of DEFAULT_SUTS list, which was leftover from v0.5. The SUTs in that list aren't really that relevant, and I think it's reasonable to assume + expect that user will specify SUT uids at the command line.
• Relatedly, you now must set at least 1 --sut <uid> arg when running the CLI.
• Modelbench does not register SUTs. I think we should keep SUT registration in modelgauge in order to avoid registering an identical SUT to multiple UIDs. Also, none of those SUTs that modelbench registered were used in v1.0 anyway.
• Testing infra improvements. I created a conftest file to register a fake SUT during setup. This removes a lot of code duplication in the tests (i.e. creating + registering dummy SUTs). And the centralization will make it easier to fully murder the ModelGaugeSut wrapper.

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[rogthefrog]

Having an easy-to-read sut ID list would be super nice.