Add tests for watch command

[st0012]

I found several things when playing with the watch command:

• It mainly uses line events to detect the change instead of the return events (I'm not sure why though). This causes the program to always stop a line after the change. So in tests we need to add an extra line after the changed line. Otherwise it doesn't stop.
• It always evaluates the expression from top-level scope. So it's not able to track an instance variable inside an object like watch @name and users need to use watch obj.name instead. I think we should improve this.

But before we do anything on it, we should have some test cases to check its behavior. That's why I open this PR.

[ko1]

Thanks.
Please file an issue for other points.