Account for subtraction underflow in order creation

[fedgiac]

This PR adds an explicit underflow check for the subtractions in the getTradeableOrder function.

This is done because a revert due to underflow during a watchtower call to getTradeableOrder would cause the watchtower to stop indexing AMM order, stopping the automated order creation on the orderbook until the order is deleted and then recreated.

Note that we're mainly concerned about subtraction overflows. We're leaving all multiplication and addition overflow there because they are only likely to occur when the amounts are so large that they are close to the amount the CoW AMM isn't able to handle by design. In the case of subtraction, an underflow could happen when the AMM reserves are extremely close to the reference price. This may occur by chance for pairs with tokens that have low number of decimals, like USDC.

How to test

New unit test. Note that I changed setUpDefaultData to getDefaultData when we set the reserves by hand because otherwise there'd be two mocks set for the same oracle/pair.

[mfw78]

Minor nit, not affecting overall logic