tests: simulate adding running services to HA with rebalance-on-start

tests: simulate adding running services to HA with rebalance-on-start

Split out from Fiona's original series, to better show what actually
changes with her fix.

Currently, a newly added service that's already running is shut down,
offline migrated and started again if rebalance selects a new node
for it. This is unexpected and should be fixed, encode that behavior
as a test now, showing still the undesired behavior, and fix it in
the next commit

Originally-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>