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]

test/cases/aws.sh

@@ -1,4 +1,12 @@
 #!/bin/bash
+
+#
+# Test osbuild-composer 'upload to aws' functionality. To do so, create and
+# push a blueprint with composer cli. Then, create an instance in aws 
+# from the uploaded image. Finally, verify that the instance is running and 
+# cloud init ran.
+#
+
 set -euo pipefail
 
 source /usr/libexec/osbuild-composer-test/set-env-variables.sh