tests: fix for pytest>=4.6.0 caplog

[ei-grad]

See pytest-dev/pytest#5312.

New pytest's logging formatter outputs the messages with a pretty indent of newlines in a single message, and this indent appears in caplog.text:

(Pdb) !print(warning)
You are adding a large directory 'large-dir' recursively, consider tracking it as a whole instead.
HINT: Remove the generated stage files and then run dvc add large-dir
(Pdb) !print(self._caplog.text)
...
WARNING  dvc.repo.add:add.py:30 You are adding a large directory 'large-dir' recursively, consider tracking it as a whole instead.
                                HINT: Remove the generated stage files and then run dvc add large-dir
...

So things like "\nsome text\n" in caplog.text are not working anymore.