umoci raw mtree-validate?

[cyphar]

At the moment, we have to use a fork of https://github.com/vbatts/go-mtree that has support for rootless operations (the fork basically just lets you switch the vfs API to fseval.Rootless if you pass -rootless). This meant that for #594 we couldn't really move pkg/fseval to internal because the fork vendors that package.

Maybe it would make more sense for us to have an umoci raw mtree-validate that we can use for tests? I do like using gomtree explicitly for validation (to avoid a bug in our validation code causing us to ignore a bug) but the lack of support for rootless is kind of unfortunate.