The Test Lists are not constrained by the data source set on the template in the same way as the actual Lists. This allows you to email a test list of any data source type, no matter the data source specified on the template.
The merge fields will not populate data in test messages. The merge field data will display correctly when sending email using the Send Final button. When the final message is sent, the merge field data will display instead of the merge field placeholders.
Additionally, when using Campaigns, the seed lists do not include the merge fields as they also are not constrained by the data source. The send test and send final options do include merge fields.