TST: improve readability using dedent in test_to_string.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Improve readability of expected outputs using dedent in ``pandas/tests/io/formats/test_to_string.py``