tests: Re-enable frustum units under GCC
tests: Re-enable frustum units under GCC

We fixed a series of issues with the dot3() operator, which is used when
computing the distance from a plane; this should improve the behaviour
of the graphene_frustum_contains_point() on older versions of GCC.

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