Most spec tests allow 42 as a placeholder value for cursor ids, error codes, lsids, readConcern.afterClusterTime. So when event callbacks are generated, convert_message_for_test checks these specific fields, converting the actual values to 42 to match what the spec will compare against.

However, in the change stream spec tests (CDRIVER-2670) 42 is allowed as a placeholder for any field. So instead of converting the command monitoring events when they're generated, change streams have a special match context flag, allow_placeholders allowing match_bson_value to interpret the value 42 to mean "match any value".

Consequently there's redundancy between what the allow_placeholders flag and the convert_message_for_test function accomplish.