Factory is an implicit return value of createPool for base pool test

[jubeira]

For base pool tests, we need to assign the factory inside createPool. This is an implicit return value, and it's not obvious for builders trying to reuse base pool test.

See https://github.com/balancer/balancer-v3-monorepo/blob/main/pkg/pool-weighted/test/foundry/WeightedPool.t.sol#L70 and https://github.com/balancer/balancer-v3-monorepo/blob/main/pkg/vault/test/foundry/utils/BasePoolTest.sol#L56.

Let's make it more explicit. I don't think we need to add the value to createPool, but maybe create another method createFactory that is specific to base pool tests would be better. Pool specializations will need to override it properly.