BUG: coerce pd.wide_to_long suffixes to ints
- [x] closes #17627
- [x] I had to change nearly all the tests which had the suffixes as strings to integers. I also added a few other tests including one for string suffixes
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I also cleaned up the finding of the var_names and substituted in some list comprehensions.