Read the existing test, run it and understand what it is asserting. The shared methods used in those tests should be in one of the following files
devtools/client/aboutdebugging/test/head.js
devtools/client/shared/test/shared-head.js
Sometimes we may already have implemented the test as a side-effect of another bug, so try to check if by any chance this has not already been ported to the new about:debugging.
Identify if the features used in the test to migrate are already implemented in the new about:debugging. If they are not, either: