Fix flake warning with bound loop var

[wbarnha]

Many PRs were getting this issue:

Run scripts/check
+ isort --check --diff --project=faust faust tests setup.py
+ black --check --diff faust tests setup.py
All done! ✨ 🍰 ✨
342 files would be left unchanged.
+ flake8 faust tests setup.py
tests/bench/base.py:98:65: B023 Function definition does not bind loop variable 'time_1st'.

The goal of this PR is to fix the error.