> FAIL: zdiff
> ===========

On this system (Alpine linux + busybox), 'diff' always produces unified output (regardless of using '-u').
It does not support 'normal' output ( https://busybox.net/downloads/BusyBox.html#diff ).

The attached patch is a suggestion to always test with unified diff.
It solves the problem on this system, but might introduce problems on other systems which don't support '-u' (or don't have sed).