test_alternator_proxy_protocol starts a node and connects via the alternator ports.
Starting a node, by default, waits until the CQL ports are up. This does not guarantee
that the alternator ports are up (they will be up very soon after this), so there is a short
window where a connection to the alternator ports will fail.
Fix by adding a ServerUpState=SERVING mode, which waits for the node to report
to its supervisor (systemd, which we are pretending to be) that its ports are open.
The test is then adjusted to request this new ServerUpState.
Fixes#28210Fixes#28211
Flaky tests are only in master and branch-2026.1, so backporting there.
Closesscylladb/scylladb#28291
* github.com:scylladb/scylladb:
test: test_alternator_proxy_protocol: wait for the node to report itself as serving
test: cluster_manager: add ability to wait for supervisor STATUS=serving