Alphametics: add two cases to test invalid puzzles (#277)
Alphametics: add two cases to test invalid puzzles (#277)

1st case to check puzzles without a solution.
2nd case to check puzzles with leading zero values.

references exercism/x-common#273
kgengler/x-common,exercism/x-common,petertseng/x-common,Vankog/problem-specifications,ErikSchierboom/x-common,jmluy/x-common,exercism/x-common,Vankog/problem-specifications,kgengler/x-common,rpottsoh/x-common,petertseng/x-common,jmluy/x-common,ErikSchierboom/x-common,rpottsoh/x-common,jmluy/x-common