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/regression-composer-works-behind-satellite-fallback.sh

@@ -1,5 +1,10 @@
 #!/bin/bash
 
+#
+# test osbuild-composer when the host system doesn't have redhat.repo file,
+# but there is a certificate that can be used to access the repositories
+#
+
 set -exuo pipefail
 
 function greenprint {