Kill subprocesses when test ends

[erik-megarad]

Background

Before we introduce webserver tests we need to be able to kill subprocesses so there aren't lingering ones between tests.

Changes

Adds a pytest fixture to kill subprocesses.

PR Quality Checklist

• I have run the following commands against my code to ensure it passes our linters:

  black .
  isort .
  mypy .
  autoflake --remove-all-unused-imports --recursive --ignore-init-module-imports --ignore-pass-after-docstring --in-place agbenchmark