Co-authored-by: Adam Johnson <me@adamj.eu>

Add test to ensure that clearing the isolation scope clears the `last_event_id`.

Users who enter a transaction without calling `start_transaction` likely intended to start the transaction, since without a call to `start_transaction`, their transaction will not get sent to Sentry. This warning message clarifies this behavior, and could help avoid the confusion that led to issue #2990.

Also, add tests to ensure the message is logged.

This reverts commit c80cad1, which appears to have introduced a regression preventing checkins from being sent when a cron job is finished.