log stdout if DF fails

ci/run-tests.py

@@ -112,11 +112,12 @@ try:
             stdin=subprocess.PIPE if args.headless else sys.stdin,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE)
-        _, err = process.communicate()
+        out, err = process.communicate()
         if err:
             print('WARN: DF produced stderr: ' + repr(err[:5000]))
         if process.returncode != 0:
             print('ERROR: DF exited with ' + repr(process.returncode))
+            print('DF stdout: %r' + repr(out[:5000]))
 finally:
     print('\nRestoring original init.txt')
     shutil.copyfile(init_txt_path + '.orig', init_txt_path)