This PR includes two changes that make gossiper much less likely to mark
nodes as down in tests unexpectedly, and cause test flakiness in issues
like SCYLLADB-864:
- fixing false node conviction when echo succeeds,
- increasing the failure_detector_timeout fixture.
Fixes: SCYLLADB-864
No need for backport: related CI failures are rare, and merging #29522
made them even more unlikely (I haven't seen one since then, but it's
still possible to reproduce locally on dev machines).
Closesscylladb/scylladb#29755
* github.com:scylladb/scylladb:
test/cluster: increase failure_detector_timeout
gossiper: fix false node conviction when echo succeeds