Fix/last line of function ignored

[limanjun99]

This PR fixes this issue.

def func():
  a = 1
  b = 2

func()

The fix is to override the user_return function of the debugger, so that we can get the frame right after b = 2 is executed but before the function returns and local variables are dropped.

Added the testcase above to executor, and tested on the frontend too.