Add tests for pseudonymisation

[stefpiatek]

Addressed

• pulled in main, quite a few changes here but if you pull in main yourself then I think those will go away?
• Broke out some of the helper methods into other files so its a bit easier to have concise test files
• Using environmental variables instead of config file! Now I understand why it was complaining about not having the yaml file, all of the services will be passed environmental variables which are read in by config so that makes it easier and tests can be run from anywhere
• Had a quick go at writing some tests for the pipeline, I think they're sufficient for our changes

Next steps assuming you're happy with these

I think this will need a couple of things for it to work:

• Add required enviornmental variables using something like os.environ, conftest.py is a classic place to do this because its run before the test. Then the env variables exist during testing with dummy values so we don't throw errors
• monkeypatch the requests.get method to return an object which has the statuscode and content fields
  • status code can always be 200
  • for a fake hash the tests are expecting adding dashes between each character and returning it encoded into bytes using utf-8

[ruaridhg]

Looks good, will add in the env vars now