Use VMware Workstation
1. Create baseline VM. "VM0"
2. Clone VM using a linked-clone "VM1"
3. Install and Test app on VM1
4. Clone VM using a linked-clone "VM2"
5. Install and Test app on VM2
This would be for a non-shared small scale test environment. If you have a lot of testers and want to open it up, then I'm using a VMware View pool that provisions new machines that are tested.
Oops, this is the VMware Player forum. backup the VM files when you have it in the base state. Do your testing and when you are satisfied restore the backup.
do you run Virtual BOX ?
in Workstation that is an implemented feature : linked clones
in VMplayer you can use linked clones too - but creating them is a bit tricky (but possible)
I wrote a howto several years ago - I can look up the link if you need it