Fix Additional Software test suite on Bullseye

sslh lacks a default configuration, which makes its systemd unit (enabled by default) fail to start, which breaks tests.

The popularity-contest package seems to be a good candidate: it asks a high-priority debconf question (just like sslh), does not ship any service that's broken by default, is tiny, and has zero dependency that we don't install already.

This MR supersedes !786 (closed).

Closes #18903 (closed)