You send it to a friend of yours that has a mac, but isn't a developer.

I hope you are being sarcastic! That’s a low quality assurance bar, even by a hobbyist’s standards. “My friend with one Mac running one OS version says it works, therefore I’m ready to release?”

I'm not. When you're a high school student releasing your first computer game on a well-tested engine, that's a perfectly valid level of quality assurance.

Not everyone can afford to keep around a lab full of macs running multiple versions of macOS.

No one says you need a bunch of Macs. People are saying that you need one, which is one more than the OP wanted to use.

I’d think any developer would want at a bare minimum two test machines: one representing the minimum system required by your application including minimum targeted OS version, and a higher end machine running the latest.

Judging my the reaction to my comment, Today I Learned it’s acceptable to release software with only cursory testing on a friend’s computer. Explains a lot about recent software quality actually...