That depends in large part on where you’re running the tests.
If you’re running tests in build (good for PHPUnit and similar), then you can leave whatever artifacts you want on disk as it is still writeable at that point.
If you’re running the tests on a complete site, which I presume is the case if you mention Diffy, you’ll want to run them in a
post_deploy hook so that the site is live. At that point the only writeable areas are the mounts you’ve defined; you’re free to make a web-accessible mount for just that purpose, though, and write your output to there. Or a non-web-accessible mount and put a tiny PHP script in front of it to control access.
There’s also nothing on the Platform.sh side that would block you from uploading test artifacts to an S3 bucket or similar if you wanted to.
Basically, you can write the test output to any number of places. Which places you want to write it to depends on your use case and the tools you’re using.