[build-bot] Fix character escapes in ignore list

Summary:
The ignore list used in production erroneously escapes the backslashes.
This can be verified by executing the following:

cd contrib/buildbot
python3   # the rest of the commands are in the python session:

f = open('./ignore-logs.txt', 'rb')
f.readlines()

Observe that the last array element has erroneously escaped backslashes:

b'err:ntdll:RtlpWaitForCriticalSection section \\\\w* "\\\\?" wait timed out in thread \\\\d*, blocked by \\\\d*, retrying\n'

This patch also converts the test instances to raw strings so that
the production and test data are testing similarly formatted strings.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7940