tests: Increase converage of graphene_quaternion_t
tests: Increase converage of graphene_quaternion_t

As measured by lcov:

                    Hit     Total   Coverage
    ----------------------------------------
        Lines:      163       187     87.2 %
    Functions:       20        20    100.0 %

ebassi/graphene,criptych/graphene,criptych/graphene,criptych/graphene,criptych/graphene,ebassi/graphene