scylladb-patched

This is a follow-up to #13399, the patch
addresses the issues mentioned there:
* linesep can be split between blocks;
* linesep can be part of UTF-8 sequence;
* avoid excessively long lines, limit to 256 chars;
* the logic of the function made simpler and more maintainable.

Closes #13427

* github.com:scylladb/scylladb:
  pylib_test: add tests for read_last_line
  pytest: add pylib_test directory
  scylla_cluster.py: fix read_last_line
  scylla_cluster.py: move read_last_line to util.py

(cherry picked from commit 70f2b093)