vectors: Correctly initialize const one vectors

We cannot use 'splat' to initialize the one vectors for vec2 and vec3
because the public initializers will set the unused components to zero,
which will lead to this equalities failing:

  graphene_vec2_equal (graphene_vec2_init (&tmp, 1.f, 1.f),
                       graphene_vec2_one ());

  graphene_vec3_equal (graphene_vec3_init (&tmp, 1.f, 1.f, 1.f),
                       graphene_vec3_one ());
tests: Add units for vector equality

Take the code coverage of the vectors units back to 100%.
tests/box: Verify inset does not yield negative widths

Coverage back to 100%.
tests: Add initial graphene_triangle_t suite
tests/quad: Improve coverage
tests: Add initial suite for graphene_sphere_t