test: run tests directly from the loopback device

test: run tests directly from the loopback device

Before, we'd copy the test tree into nspawn-root, and run the tests from there.
This is OK, and doesn't actually take much extra time. But it uses quite a lot
of extra disk space. So let's make things a bit more efficient by running
directly from the image file.

We still run the unprivileged nspawn tests from a copy. Once the kernel
implements fs shift, we can do away with that too.