DOC: Moved PANDAS_TESTING_MODE tip to .travis.yml (#30694)
Added comments for travis.yml for PANDAS_TESTING_MODE from tips and tricks section of wiki

- [X] xref #30964
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
