Stop ignoring `Local variable name is assigned to but never used (F841)` for tests in`flake8`

**Problem**
When developing, I was surprised that `flake8` was not alerting me to unused variables I had left in the code after making some changes. I understand the original reasoning to ignore `F841` as laid out in #4888 but I would argue that it helps with refactoring existing test code, making sure that tests are not bloated by legacy code.

**Proposal**
TL;DR: Boy-scout and flip the switch later.

I do **not** propose removing `F841` from the ignore list and dealing with all the new issues at once. Instead, I would like to remove those over time and flip the switch afterward. The main goal of this issue would be to reach a consensus that we want to remove `F841` from the ignore list for tests or highlight any potential issues.

**Possible Concerns**
One concern brought up by @graingert in an offline discussion was that some unused variables might be used for testing the GC by assigning something to an unused variable to keep it alive. For these cases, I would argue that it is beneficial to add an in-line comment to those unused assignments ignoring `F841` and more importantly, explain why this statement is needed. Otherwise, other developers might be tempted to remove the supposedly useless assignment at a later point in time.