[ETHEREUM-CONTRACTS] improve testability of custom (mostly pure) SuperTokens

[d10r]

During the development of GoodDollar SuperToken, the issue of testability came up. It's a custom Pure SuperToken which overrides some of the stock SuperToken logic - it also deploys its own - slightly modified - version of SuperToken logic.
Because of this changes, the project owner wanted to run the SF test suite against the token in order to feel more confident about not accidentally breaking something.
It ended up being easier to add the GD contracts to a copy of the SF repo than to port tests.

Task: investigate how we could make it easier for cases like this to get enough test coverage. Example usage could then be shown in https://github.com/superfluid-finance/custom-supertokens/.

[hellwolf]

how's the testability now?