Replace the fixed Process.sleep waits in the import, members-PDF and
field-visibility tests with event-based / bounded-poll waits on the
observable condition, removing a known flakiness vector.
DateTime.from_iso8601/1 returns {:ok, datetime, offset}, so the two-tuple clauses never matched and datetime cells fell through to the naive-parse fallback. Matching the real shape routes them through the intended DateTime path; UTC values render identically.
