WIP: test_upstream_compatibility: handle pkgrel offset

A few pmaports are generated from Alpine's aports, using the same pkgver and pkgrel. With pmbootstrap!1806 (closed), we introduce a pkgrel offset, that allows increasing the pkgrel independently of changes in Alpine's pkgrel. The original pkgrel is stored in _pkgrel, so adjust the upstream compatibility test to read _pkgrel for comparing, if it is present.

Setting to WIP until pmbootstrap!1806 (closed) is merged. CI is expected to fail until then.

EDIT: This check only runs on master, that is why CI does not fail here.