Obviously if your friends have a Mac use that to test, but there is a cheaper alternative than buying a Mac -
I've not used it yet, but when I have a game exported for Mac that's what I plan to use to test it.
If you're a student, you can get access for 3 hours a day for as little as $16 a month
There's also a pay as you go option for non-students: 30 hours for $30.
I don't know if there is a linux equivalent.