sdk: fix test fixture typing issues

[tmigone]

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          As mentioned in our call, returning `contracts as L1FixtureContracts` or `as L2FixturContracts` would solve a lot of the type wrangling in the tests, but can be addressed in a separate PR

Originally posted by @pcarranzav in https://github.com/graphprotocol/contracts/pull/872#discussion_r1404587658