You probably know this, but unless you have a Mac (or some “self-build,” aka Hackintosh–and that can get tricky, especially with a just released OS), no OS you manage to find will install. Any OS needs the appropriate drivers present in the Apple hardware. Since 10.9 (Mavericks) new OS versions have been free from Apple. The latest is 10.11, El Capitan, just released, and it’s available free for download from Apple by starting here. https://itunes.apple.com/us/app/os-x-el-capitan/id1018109117?mt=12
Unfortunately, as soon as a new OS is released, Apple pulls the current free version, which, until a few weeks ago, was 10.10.5 Yosemite. I usually like to wait until at least one point update has been released, which fixes the most obvious bugs, but since you just want to have this for testing purposes, maybe this will do.
If push comes to shove and you want Mavericks 10.9, I can put that up on File Dropper for you, in maybe 7 or 8 splits, but you need a fast connection to get that because FD has problems when downloading with a slow connection–things end up downloading corrupt or just partially, if the connection isn’t relatively fast. Or maybe you can suggest a better, preferably free, alternative that doesn’t involve too much hassle for signing up, if signing up for such a site is required.