Merge #12853: qa: Match full plain text by default

Summary:
faace13868 qa: Match full plain text by default (MarcoFalke)

Pull request description:

Instead of escaping all full plain text error strings, just compare their strings by default.

Tree-SHA512: 42e28f55105eb947ac6af6ce4056f0ec0f701d85f1c2a38b35ab777bbdf2296bdb79639c345621b8adc03a98b28c7630ded9a67b8b04a48e2c3a49d598ecdcd7

Backport of Core PR12853
https://github.com/bitcoin/bitcoin/pull/12853/files

Test Plan: test_runner.py

Reviewers: #bitcoin_abc, deadalnix, Fabien, nakihito

Reviewed By: #bitcoin_abc, Fabien, nakihito

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3914