Create virtualized test harness

[albertofaria]

Tests currently consist simply of launching VMs using crun-vm and checking whether certain configurations work properly by inspecting the guest. This makes it difficult to test features like block device pass-through or PCI device assignment, which depend on the host environment and may require privileges.

We should run tests inside a VM instead, using crun-vm itself to launch that outer VM. Tests would then effectively launch nested VMs. This would allow us to have a well-defined test environment and would preclude the need for elevated privileges to run the tests.

[albertofaria]

Accomplished in #44.