feat: Add integration test setup

Add an integration test setup, which creates required virtual machine
images with the help of `mkosi` and runs `caterpillar` in them using
QEMU.
After creating the machine images once, the test setup can reuse them
and resets them before each test run.

The test setup can only be run serially, as otherwise the virtual
machine images have to be copied to temporary locations, which may
require an unknown amount of space during test runs.
Therefore the default recommendation is to not run them by default:

```sh
cargo test -- --skip integration
```

The integration tests are also switched off for the the git hooks
integration.

Signed-off-by: David Runge <[email protected]>

.ci/check.sh

@@ -7,6 +7,6 @@ set -euxo pipefail
 
 cargo fmt -- --check
 cargo clippy --all -- -D warnings
-cargo test --all
+cargo test -- --skip integration
 codespell
 reuse lint