MDL-47420 Grades: Get unit tests passing on natural grading branch

Part of: MDL-46576
MDL-47253 core_grades: set def aggregation method to Natural

Part of: MDL-46576
MDL-47253 core_grades: display scale in Natural setup

Part of: MDL-46576
MDL-47418 Grades: Fix debugging in behat test.
MDL-47430 core_grades: Handle negative grademin with natural weighting
MDL-47430 core_grades: Adding behat test for alternative mingrade