[rsyncable] Fix test failures

[rsyncable] Fix test failures

Test failures showed up on the daily cron job. They didn't show up
in CI because the condition is somewhat rare, and didn't trigger
during the CI tests.

This PR fixes up the logic in `findSynchronizationPoint()` to correctly
handle the edge case. It also un-comments an assert that helps catch the
issue, and verify that rsyncable mode is calculating the correct hash.

After the fix, the test that failed passes:

```
./zstreamtest --newapi -t1 --no-big-tests -s9680
```