Let's see if this solution can meet your needs.
You could add some more options to your menu bar, let's say "All" and "Selected" items within both Test e Repeat items. The callback for "All" items will run your tests as you're already doing. The callback for "Selected" items will show the popup for the operator to select the desired tests and run them.
I'm attaching a modified UIR with this solution.
The callback for Ok button will detect the state of checkboxes (GetCtrlVal...) and pass it to the testing routines.
It's not necessary to maintain the popup panel once the status of checkboxes have been read, so you can DiscarPanel before starting the tests (the testing routines will have thei own panel, I suppose...).
Roberto