luhn: tweak canonical tests (#547)
luhn: tweak canonical tests (#547)

* luhn: tweak canonical tests

* luhn: further adjustments to canonical test suite

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