[Seaside] Test Runner question

[itsme213]

I want to customize TestRunner with a class-side method so it opens with 
just my classes from MyCategory in it e.g.

TestRunner onMyStuff.

I tried as below but it still opens with every test class in the entire 
image. Help?

onMyStuff
 "for testing MyStuff"
 | tr |
 tr := self new.
 tr categoriesSelectedFrom: { 'MyCategory' }.
 ^ ToolBuilder open: tr

Sophie