[Cashtab] Add test cases for one-to-many xec sends

Summary:
Depends on D14575

Add unit tests for sendXec on multi-output txs.

Tx raw hex imported to ElectrumABC to verify txid and fee. Note: the p2sh fee is 1.03 sat/byte due to p2sh not being explicitly covered in ecash-coinselect ... the potential available benefit to nailing down p2sh tx sizes is not material for most use cases, probably why it was not done in original coinselect lib

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14576