[Cashtab] Mock getRawTransaction API calls in unit tests

Summary:
T2577

Earlier review missed this API call, which only applies for received txs. This diff mocks the API call, patching longstanding bug in unit tests covered by replyAddress: null from earlier error.

Paves the way for D11672

Test Plan:
Review the new mocks
npm test

Reviewers: emack, #bitcoin_abc

Reviewed By: emack, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11777