stop tests for the runner's drain !352

The real stop test from #184's "under any answer" list, plus the two cheaper checks around it.

• TestRunnerDrainsOnSIGTERM: process-level drain, runs everywhere.
• TestRunnerDropInLetsTheDrainHappen: parses the drop-in for KillMode=mixed and a TimeoutStopSec that outlasts -timeout plus the report retries.
• TestRunnerDrainUnderSystemd: the runner as a transient user unit via systemd-run, stopped with systemctl --user stop, under the drop-in's mode (build succeeds) and under control-group (it does not). Skips without a systemd user manager, so it skips in CI's container; it has not yet been run under real systemd.

Ref #184, Ref #179