Where can I download various versions of MS Excel for free for testing purposes?

I am making a web app with a "Download as CSV" feature. I was hoping people using MS Excel would be able to open my CSV files by double-clicking on them, but it seems most versions of Excel don't handle utf-8 CSV files correctly out of the box (all other spreadsheet programs I've tried can do this, only MS Excel is problematic). So my plan B is to change the feature to "Export to Excel", which will offer a xlsx file for download instead of a csv file. This seems to work (there is no utf-8 bug for xlsx files) in the versions of Excel that I have tried so far. Of course, I'd like to verify on a few different versions of Excel on a few different versions of Windows and also on Mac. On http://modern.ie, I found a few virtual machines I can download with Win-7, Win81 and Win10. This is good but now I need a place to get all these different versions of MS Excel for free.

Where can I find the most commonly used versions of MS Excel for free for testing purposes?

Obviously I'm looking for a 100% legal solution to my problem here, illegal download links won't get a checkmark or any up-votes from me.

How do you all usually make sure your xlsx files will be correctly opened in all versions of MS Excel?