tests: Fix potential wrong output when two tests fail in parallel
tests: Fix potential wrong output when two tests fail in parallel

This should actually never ever happen, but just in case it does, we can
protect against this case a little.
The default behavior is that the comparison succeeds and we don't
actually need to invoke diff at all, so the extra allocations will not
slow down the tests.
CC: #222

ximion/appstream,ximion/appstream,ximion/appstream,ximion/appstream