I'm currently running OS X 10.8.5 and I would like to set up a virtual machine that runs OS X 10.9. (The other way around would be fine too. I would upgrade to OS X 10.9 and run OS X 10.8.5 in a virtual machine.)
Use case: I'm a Mac developer and I would like to test my mac app on different operating systems versions. If I would develop my app on OS X 10.9 there would be no way to know if the app runs well on OS X 10.8.
Do you know any tool that accomplishes that? It shouldn't be a licensing problem, because I have already "bought" licenses for OS X 10.6, 10.8, 10.9 and 10.10-beta2.
What I tried so far...
I found this question which basically asks the same thing, but I think its not a duplicate, because that question is from 2010, things probably have changed since then, and this question is not about OS X 10.5 or 10.6. (At the time of this writing we have 2014.)
Another option would be to install to a separate partition on an external hard drive, but that's less flexible than a virtual machine.
Other acceptable solutions
For me, using a virtual machine is optional. I'm really just searching for the easiest process to verify wether my software runs smoothly on different OS versions, so other solutions - not based on virtualization - are welcome too!