Creating a variant actually create an extra one via the associated
product, as it will create a "standard variant".
As far as I can see there is no way around it, but it should be fixed
once the Product refactor is finished, and product becomes product
group.
Added a comment on the variant factory to explain the problem.
It's not ideal as it will slow down the test suite a little, but I think
it's better to write the code the way you would expect it, and it will
eventually get fixed.
f60a79437e