gh-135272: Activate doctests on turtle module

[adorilson]

This PR is a WIP to fix #135272

I'd like reviews and comments before I continue.

Also, I need some help:

1. I added a load_tests function to include doctests in unittest, and already fixed some ERRORs on tests (8 remaining), the majority were just adding import turtle and instantiating a turtle object. Is it possible to avoid repeating it in every doctest?

2. I mocked the mainloop function. But maybe it was the right way. How is it?

3. Any recommendations about how to mock other functions that wait for a user action (num/textinput, ...)?

4. The optionflags=doctest.ELLIPSIS|doctest.NORMALIZE_WHITESPACE, options seem not to work properly. For instance, (0,0) is saw different of (0, 0) (common on tests with position()). (Or I did understand how these options work.

This PR is related to #111704

• Issue: Create tests for turtle module #135272