fix: allow YATT to insert into and check contents of DDL sources

[jzaralim]

Description

Fixes #6113 and #6058

The issue is that the testing tool works by creating a TopologyTestDriver for every query, so when doing insertions and checking values, it looks for drivers. DDL sources are not queries though, so YATT ends up not finding a driver. The fix here is to register a driver with an empty topology for every DDL source.

Testing done

Added a test to the testing tool test

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[jnh5y]

LGTM.