Add RUMBA_VERIFY test
This adds a test for the RUMBA_VERIFY
functionality introduced in BuildGrid/rumba!35 (merged)
It's run twice. Once with buildbox-run-hosttools
where the digests are expected to match. And once with buildbox-run-userchroot
where the digests are expected to mismatch because the userchroot sandbox uses a different toolchain.
e2e CI run to validate this addition: https://gitlab.com/BuildGrid/rumba/-/jobs/2504330708