[backport#16917] tests: Move common function assert_approx() into util.py

Summary:
96299a9d6c0a6b9125a58a63ee3147e55d1b086b Test: Move common function assert_approx() into util.py (fridokus)

Pull request description:

To reduce code duplication, move `assert_approx` into common framework `util.py`.

`assert_approx()` is used in two functional tests.

Backport of Core PR16917

Test Plan:

test_runner.py wallet_avoidreuse wallet_groups

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6587