It seems like playtest should be written together as a compound word, but when I do so, autocorrect adds a space. If I insist, I'm rewarded with an ugly red squiggly.
Writing "play test" makes the activity seem less specific. Aesthetically, it's just not as appealing to me. Maybe I'll start using "play-test". It has the feel of a compound without being auto-incorrected.
So this is the reason while you'll see a variety of spellings for "play-test". Regardless of how you write it, the important thing is to do it and do it and do it again.
What do you think is the best way to write "play-test"?