MockS3_PINGDiscoveryTestCase.testDiscovery is flaky in CI

[rhusar]

MockS3_PINGDiscoveryTestCase.testDiscovery intermittently fails in CI with:

member count ==> expected: <5> but was: <4>

Fails non-deterministically on JDK 17 (Ubuntu/macOS); rerunning passes. Likely a timing issue on slower runners — the discovery timeout may need to be increased.

Example: https://github.com/jgroups-extras/jgroups-aws/actions/runs/25569466993/job/75061215084 (surfaced in #576)