test: add test purpose

Most test scripts don't have any documentation regarding it's purpose,
although it can be guessed by the code. There's value in adding this
small comment.

[skip-ci]

tools/libvirt_test.sh

@@ -1,6 +1,11 @@
 #!/bin/bash
 set -euo pipefail
 
+#
+# tests that guest images are buildable using composer-cli and and verifies 
+# they boot with cloud-init using libvirt
+#
+
 OSBUILD_COMPOSER_TEST_DATA=/usr/share/tests/osbuild-composer/
 
 # Get OS data.