Create test log that captures step commands and output

In `sev_verify`, add more log files. Save them into the appropriate `artifacts/` directory. This will aid in debugging when a VM launch errors or hangs, or when a test step fails with unclear reasoning.

Ideally, we'd have:
- file with step commands and output
- file for QEMU command + QEMU serial output (useful for if the guest doesn't boot all the way)
- file for guest journald (probably best to pull it over vsock)
- support for multiple VM launches (so VM logs don't get overwritten if there are multiple launches in the same test)