ADD test cases for sudoku::find_cell_value_by_exclusion

The function will run find_feasible_values first, but will also try to
determine the cell value by exclusion:
If one of the feasible values is not feasible for any cell in the same
row or column or block, that number is the cell value, and other
feasible values can be discarded.

The test cases uses easy6 and medium16 as examples to test the
identification of this situation
ADD more test cases for sudoku::find_cell_value_by_exclusion