Update CI to load current Kubo and run loopback tests

The current `TestFixture.Ipfs` implementation for running unit tests currently just creates an `IpfsClient` instance with the assumption of an IPFS local server to connect to. Update the test code to:

- Download latest Kubo and run locally in loopback mode.
- Ensure Kubo's storage is in a fresh directory for each test for cleanliness. This may imply stopping and starting the server between each test. Consider using https://github.com/microsoft/CopyOnWrite/blob/main/tests/utils/DisposableTempDirectory.cs for easy unique dir create and cleanup for each test.
- Validate this runs successfully in CI on GitHub Actions VMs.

Note this item will probably illuminate a lot of missing or out of date functionality. File incompatibilities as new issues.