run-tests.py: print diff if reference output not existing.

tests/run-tests.py

@@ -182,6 +182,8 b' def run_one(test):'
         f = open(ref, "r")
         ref_out = f.read().splitlines()
         f.close()
+    else:
+        ref_out = ''
         if out != ref_out:
             diffret = 1
             print "\nERROR: %s output changed" % (test)