test/ostree: fail on unsupported distros

If the ostree test was run on an unsupported distro, it failed but with a
very weird error message. This commit makes the test fail fast and with a
nice message.

test/cases/ostree.sh

@@ -29,7 +29,9 @@ case "${ID}-${VERSION_ID}" in
         # When 8.3 was released, it wasn't available on all RH internal
         # mirrors, therefore the Boston mirror is hardcoded.
         BOOT_LOCATION="http://download.eng.bos.redhat.com/released/rhel-8/RHEL-8/8.3.0/BaseOS/x86_64/os/";;
-    *) ;;
+    *)
+        echo "unsupported distro: ${ID}-${VERSION_ID}"
+        exit 1;;
 esac