Refactor deployment/test helpers

[EndymionJkb]

As noted in the description of #2078 , the test helpers - WeightedPoolDeployer in particular - have, with the proliferation of pool types, grown into "unholy abominations" that are doing too much. We have moved the solidity codebase from generic base contract with lots of special casing toward encapsulating pool-specific functionality within each contract, and could do much the same here.