Configuration-dependent regression testing

[sim642]

Currently we use a very crude hack (update_suite.rb runs SKIP tests when running by group name) to execute apron analysis tests. This is inconvenient, because that's not included in the default run (and hence is easy to forget). In the CI those are explicitly specified in a separate step.

Also regtest.sh ignores SKIP and allows running those tests, so the hack is quite usable, but confusing. And there's no way to truly skip an apron analysis test then.

This could be supported in a nicer way.