open stepper, make sure it works for (+ 3 4).

make sure that stepper button appears and disappears as necessary when
language level changes.

Make sure that you get a warning when you change the underlying program, and a
warning when the program window disappears.
** closing a tab does not trigger the error message that it should...**

Try stepping backward and forward through programs with correct and erroneous
(syntax errors, runtime errors) executions.

Test programs with syntax errors

Try programs which print snips (print-convert-test.ss)

try programs that contain test cases; make sure that the popups behave sensibly.

Try jumping to the end on a program with an error.
** jumping to end when already at end doesn't behave properly ("no step
   matching that criterion")
