`test_crashed_node_substitution` intermittently failed:
```python
assert len(gossiper_eps) == (len(server_eps) + 1)
```
The test crashed the node right after a single ACK2 handshake (`finished do_send_ack2_msg`), assuming the node state was visible to all peers. However, since gossip is eventually consistent, the update may not have propagated yet, so some nodes did not see the failed node.
This change: Wait until the gossiper state is visible on peers before continuing the test and asserting.
Fixes: [SCYLLADB-1256](https://scylladb.atlassian.net/browse/SCYLLADB-1256).
backport: this issue may affect CI for all branches, so should be backported to all versions.
[SCYLLADB-1256]: https://scylladb.atlassian.net/browse/SCYLLADB-1256?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQClosesscylladb/scylladb#29254
* github.com:scylladb/scylladb:
test: test_crashed_node_substitution: add docstring and fix whitespace
test: fix race condition in test_crashed_node_substitution
b708e5d7c9