Expose unsupportedFeatures from buildkit's integration tests

[jedevc]

👀 See #1844 (comment).

BuildKit upstream has a utility for skipping tests if certain functionality is not implemented by the chosen backend using integration.CheckFeatureCompat. We should rework this upstream, so that we can use this function inside buildx, and expose the driver features (and maybe other similar keys) to tests. (We should probably also allow for a variant of the function that returns a boolean instead of immediately skipping the test).