test: unit coverage for poll legacy

[mfw78]

Background

It's frustrating working on the polling legacy logic as it is detailed and lacks unit testing, inhibiting robustness / trustworthiness.

Details

Implement a test harness for unit testing the legacy polling logic. This should be accomplished using jest test harnesses with mock for the getTradeableOrderWithSignature, and unit testing for customErrorDecode.

Acceptance criteria

• [x] Test coverage of customErrorDecode
• [ ] Happy path test coverage of getTradeableOrderWithSignature

[anxolin]

I wonder if we should just do this directly in the SDK as part of creating the GenericConditionalOrder