tests: remove unused mix_stderr param (#549)

Author: jirikuncar

tests/integration/dogshell/test_dogshell.py

@@ -78,7 +78,7 @@ def run(args, stdin=None, check_return_code=True, use_cl_args=False):
             ] + args
 
         with capsys.disabled():
-            result = runner.invoke(main, cmd, input=stdin, prog_name=cmd[0], mix_stderr=True)
+            result = runner.invoke(main, cmd, input=stdin, prog_name=cmd[0])
         return_code = result.exit_code
         out = result.stdout_bytes
         err = result.stderr_bytes