[cashtab] Avoid loss of precision eslint error

Summary: There are 3 occurrences of eslint no-loss-of-precision in the validation tests, one of them is deliberate and the others can be fixed.

Test Plan:

eslint src/utils/

Check there is no error

npm test

Check the validation tests pass

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D12923