Restore tests and update them from 19.09

This PR starts with a selective revert of b8661825, that lost most of the tests/ folder.

I also bundled the following improvements in the branch, each reviewable in its own commit:

• Update clamav.nix suite to use local fixtures instead of downloading them from gitlab
• Increase dhparam value to make dovecot start OK on 19.09 and unstable
• Update the "dkim has user-specified size" test for 19.09 and unstable
• Rework the gitlab CI config to run tests on 19.03 + 19.09 + unstable