Remove unnecessary files generated when running tox locally

[matthewhegarty]

Problem

Fixes issue #1424
Coverage files and 'response.html' get generated when running locally

Solution

• Removed the unused write to response.html
• Added an Env var which gets set in CI only, and which indicates that coverage data should be generated.
• Added a new script runtests.py which adds the COVERAGE env var if present

Acceptance Criteria

• Tested locally
• Tested in CI

[pokken-magic]

Dude thanks so much for doing this and for knowing all this minutiae that makes things flow.