More test cleanups, in the spirit of 49b3a2641

Deliberatey not fixing up everything under the same consistency umbrella: some
of the tests can not be currently executed (lack of pre-set RDBMS), and
moreover there are additional pieces missing to make this fully "clean"

Just merging outstanding work from some time ago, to make subsequent commits
easier to apply
Switch optdeps to a croak import
Rewrite optdeps to accept a list of groups

Also tighten up the optdep spec and handle conflicting versions saner
Changes to xt/optional_deps.t best viewed under -w