We hand-start every server instance the tests need, so drop a policy-rc.d
that tells invoke-rc.d/deb-systemd-invoke not to start services at all. The
packaged redis-server unit would otherwise race our own primary for
127.0.0.1:6379 on any run where WSL came up with systemd.

That also sidesteps an install hang we saw with the 8.10.0 package: its
redis.conf lost "supervised auto", so redis never sd_notify()s, the
Type=notify unit never leaves "activating", and the postinst's
"systemctl start" blocks (indefinitely, since redis then can't save its RDB
on SIGTERM and the unit disables the stop timeout). That is an upstream
packaging slip which they are already fixing, so this isn't a permanent
change in behaviour from 8.10 - but not starting the service is the right
thing for this job regardless.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

This PR is best summarised by docs/Failover.md