REFACTOR name change

to use the recursive sudoku solver
DELETE sudoku::solve_sudoku_simplistic

because it is no longer used
REFACTOR relocate test functions

move the unit tests into a file test_sudoku_functions

user acceptance tests (i.e., test the correctness of solutions) will be
moved to a separate file
ADD test_sudoku_solutions for user acceptance tests

Add more tests, now there is a total of 21 (1 given by the challenge, 20
from online: 9 easy, 8 medium, 3 hard)
Improve documentation of sudoku::solve_sudoku