test: Properly deserialize integers in little-endian

Summary:
This fixes a bug in a functional test. The bug was detected after Core started running CI tests for a big endian platform (see PR17591 and PR17599).

This is a partial backport of Core PR17599
https://github.com/bitcoin/bitcoin/pull/17599/commits/fad9fdbea5dfb19328282afda9588edc6f1d0ddf

Test Plan: ninja && test/functional/test_runner.py interface_rest

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8283