## Problem
`test_sharding_failures` is flaky due to interference from the
`background_reconcile` process.
The details are in the issue
https://github.com/neondatabase/neon/issues/12029.
## Summary of changes
- Use `reconcile_until_idle` to ensure a stable state before running
test assertions
- Added error tolerance in `reconcile_until_idle` test function (Failure
cases: 1, 3, 19, 20)
- Ignore the `Keeping extra secondaries` warning message since it i
retryable (Failure case: 2)
- Deduplicated code in `assert_rolled_back` and `assert_split_done`
- Added a log message before printing plenty of Node `X` seen on
pageserver `Y`
e6a404c66d