Read at or before the stable timestamp in the CppSuite.

When committing a transaction in Wiredtiger if the first_commit timestamp is older than the last_active_read_timestamp the commit will be rolled back, with the caller receiving EINVAL. However as a result of a recent change WiredTiger will also abort in diagnostic mode. This has affected the CppSuite as it runs in diagnostic mode and doesn't do much in the way of timestamp checking.

To avoid hitting this abort we can ensure that readers read at or behind the stable timestamp.