Add test debug command "make snapshot"
As a test developer of long running tests I want to create intermediate qemu snapshots to speed up test development
- qemu snapshots can be created using a command from tests
- a new job can continue from the snapshot using "SKIPTO=…"
- research how qemu snapshots are created
- add command to testapi (or "test debugging api"?) to make snapshot at specific point
- extend documentation
example: "livecdreboot" is a long running test. It would be helpful if snapshots could be created e.g. when the "reboot now" window is detected (and stopped).