test: Add bare-bones integration tests for practices
Added a GUT test that iterates over the practices and runs the given solution.
There are some tests that fail and it might be due to how the UIPractice._practece_completed flag is set. For now I'm emitting a test_student_code_completed signal at the end of UIPractice.test_student_code().
Other tests, like res://course/lesson-7-member-variables/draw_rectangle_at_position/TestsDrawingRectangles.gd fails to run and crashes GUT probably because it needs the scene to set up some variable

This needs further investigation.
The script is located at res://gut/integration/test_course.gd.
I just re-tested the PR, and many code solutions fail, even though they correctly work when running a practice as a user. Example: lesson 3's practices.
Could you look into this?
Should be fixed now