The AddToCartFormTest is broken

Working on the attribute weight issue uncovered a number of issues in the add to cart form test.
It creates attribute fields without using the attribute field manager, which results in the caches not being cleared.
This creates a false pass in testMultipleVariationsMultipleAttributes().

There's also a problem in testAttributeDependencies, where we setup attributes as Size -> Color, but then test Color -> Size.
Using the attribute field manager fixes the ordering, which causes the test to fail.

Plan for fixing this:
1) Start using the attribute field manager
2) Fix the assertion in testMultipleVariationsMultipleAttributes()
3) Change the expected ordering in testAttributeDependencies() to be Color -> Size, matching the assertions.