[test] feature_init: prevent UnicodeDecodeError errors when opening log file

Summary:

open log file as a raw byte stream (fixes core#23989)

The log file can be read partially while some emoji is being written, causing a UnicodeDecodeError. It did not make sense to decode the file anyway, as the file content is not being used. We read the file only to count the log lines.

This is a backport of core#24039

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12617