Move integration tests from Kubo to Boxo

[guseggert]

Currently most tests that exercise the integration of Boxo components are in Kubo. This causes Boxo releases to take longer and be more complicated, since changes have to be tested in Kubo before the release. It also creates a much longer feedback loop on Boxo changes.

Many of the "sharness" tests and integration tests in Kubo should be integration tests in Boxo, and Kubo tests should be reduced to testing that we are using Boxo correctly and that Kubo-specific features work correctly.

This issue needs to be scoped out more, with a specific "definition of done".