tests/image: add booting tests

This commit makes the osbuild-image-tests binary doing the same set of tests
like the old test/run script.

Changes from test/run:
- qemu/nspawn are now killed gracefully. Firstly, SIGTERM is sent.
  If the process doesn't exit till the timeout, SIGKILL is sent.
  I changed this because nspawn leaves some artifacts behind when killed
  by SIGKILL.
- the unsharing of network namespace now works differently because of
  systemd issue #15079

vendor/modules.txt

@@ -84,6 +84,7 @@ github.com/stretchr/testify/assert
 golang.org/x/net/http/httpproxy
 golang.org/x/net/idna
 # golang.org/x/sys v0.0.0-20200212091648-12a6c2dcc1e4
+golang.org/x/sys/unix
 golang.org/x/sys/windows
 golang.org/x/sys/windows/registry
 # golang.org/x/text v0.3.0