[pydantic > 2] [tests] [polyfactory] Remove custom value for `course_id`

## Bug Report

In `test_models_edx_converters_xapi_video_ui_load_video_to_video_initialized`, we assign a custom value to `event.context.course_id ` (`"course-v1:a+b+c"`). **This should not be necessary.** Currently removing leads to an error. This may be due to a regex wrongly interpreted by polyfactory.