[tests] simplify binary and hex response parsing in interface_rest.py

Summary:

We use assert_greater_than_or_equal(), since the hex response contains
an extra b'\n' traling byte.

Partial backport of core PR12766 (commit 55efc1c)
https://github.com/bitcoin/bitcoin/pull/12766/commits/55efc1f62cef7e19efcb58ba68fbc2053bfa19a0

Depends on D3769

Test Plan:

./test/functional/test_runner.py interface_rest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3770