[Cashtab] [p3 normalizeBalance fix] Add unit tests

Summary:
T2601

Part 3 in a series of diffs to improve calculation of wallet balance from utxo set

Depends on D11844

Add unit tests for the normalizeBalance function.

Note: One mock had legacy balances in BCHA units and was updated to XEC.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11845