[CI] Improve the error snippet in diffs when a test fails

Summary:
Looking for the build detail don't work anymore since the logs are
captured by teamcity due to the junit output. But the test logs can now
be easily retrieved and the message improved.

Test Plan:

pytest

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7982