QA: Avoid re-reading config.ini unnecessarily
Summary:
BitcoinTestFramework.main already loads and stores config.ini on the object itself; just access that instead of re-reading the file to check for features
Backport of Core PR15895
Test Plan: ninja && ninja check-functional
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7777