Failed to delete the database: TestDB. ErrorCode: 400 ErrorMessage: Database ‘TestDB’ cannot be deleted because it is used as a sync metadata database which still contains sync groups and/or sync agents.
After some digging around I remembered that I tried to setup a data sync group where this database was involved in bi-directional synchronization with another Azure SQL Database. The other database was deleted a long time ago and clicking on the sync option within the portal confirmed that nothing was actually ever connected or ready.
Arun walks us through the wrong way before getting us down the right path. I recommend the latter.