test: Use self.wait_until over wait_until_helper

Summary:
wait_until_helper is a "private" helper, not intended to be used directly, because it doesn't scale the timeout with the timeout factor. Fix this by replacing it with a call to self.wait_until, which does the scaling.

Backport of core#27226

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13527