Fail functests if refusing to run tests

We made this non-failing so tests could pass before
I92299b2f59aa8e7de8995a6294bf6c88bbd4cdc5 merges. We should make this a
test failure.

Change-Id: I4dd38c835d6fb397bbd0475120a4b0f2ab0290a8
Depends-On: I92299b2f59aa8e7de8995a6294bf6c88bbd4cdc5

tests/run_output_format_test.sh

@@ -63,7 +63,7 @@ test_formats="tar raw qcow2 docker aci"
 for binary in qemu-img docker ; do
     if [ -z "$(which $binary)" ]; then
         echo "Warning: No $binary binary found, cowardly refusing to run tests."
-        exit 0
+        exit 1
     fi
 done