test: Future-proof reader conversions tests.
test: Future-proof reader conversions tests.

Query time must be fetched after populate. If compaction is executed
during populate it may be executed with timestamp later than query_time.
This would cause the test expected compaction and compaction during
populate to be executed at different time points producing different
results. The result would be sporadic test failures depending on relative
timing of those operations. If no other mutations happen after populate,
and query_time is later than the compaction time during population, we're
guaranteed to have the same results.
Message-Id: <5ac5ccd053fbbff27bcd60e6824204571b226169@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla