Apply fixes from ucam-observe-remote-server

Adds .flake8 file to the boilerplate.

Corrects error in gitlab CI configuration which causes tests to run twice.

Moves tests module to be a part of the root package - in line with project style throughout DevOps.

Part of uis/devops/ucam-observe-remote/ucam-observe-remote-server#1

template/.gitlab/webapp.gitlab-ci.yml

@@ -23,6 +23,8 @@ python:tox:
       - PYTHON_VERSION: "3.11" # Make sure this matches the version in pyproject.toml and the Dockerfile.
   variables:
     TOX_ADDITIONAL_REQUIREMENTS: "poetry>=1.7.1"
+  rules:
+    - !reference [build, rules]
 
 container:pytest:
   needs: [build]