feat: add three new etcd cold boot recovery tests

Add three new test cases to validate etcd cluster recovery from cold
boot scenarios reached through different graceful/ungraceful shutdown
combinations:

- Cold boot from double GNS: both nodes gracefully shut down
  simultaneously, then both restart (full cluster cold boot)
- Cold boot from sequential GNS: first node gracefully shut down, then
  second node gracefully shut down, then both restart
- Cold boot from mixed GNS/UGNS: first node gracefully shut down,
  surviving node then ungracefully shut down, then both restart

Note: The inverse case (UGNS first node, then GNS second) is not tested
because in TNF clusters, an ungracefully shut down node is quickly
recovered, preventing the ability to wait and gracefully shut down the
second node later. The double UGNS scenario is already covered by
existing tests.

This change also includes skipping etcd recovery tests when cluster is
unhealthy. Previously, the etcd recovery tests would fail if the cluster
was not healthy before the test started. This is problematic because
these tests are designed to validate recovery from intentional
disruptions, not to debug pre-existing cluster issues.