Tests/db backend tests #370

[Reillyhewitson]

This PR will close #370

Description

Added tests to mirror the existing tests on the ICAT backend in order to improve coverage for the DB backend

Testing Instructions

Add a set up instructions describing how the reviewer should test the code

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?
• {more steps here}

Agile Board Tracking

Connect to #370

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@22141b5). Click here to learn what that means.
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #385   +/-   ##
=======================================
  Coverage        ?   96.90%           
=======================================
  Files           ?       39           
  Lines           ?     3331           
  Branches        ?      313           
=======================================
  Hits            ?     3228           
  Misses          ?       76           
  Partials        ?       27           

Impacted Files	Coverage Δ
...ateway_api/src/datagateway_api/database/helpers.py	95.78% <100.00%> (ø)
...gateway_api/src/datagateway_api/database/models.py	97.27% <100.00%> (ø)

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[MRichards99]

An additional 2% is really good work. There's a few things that I've noticed but the bulk of it looks really good - I know tests isn't the most interesting work so thanks for getting on with it :)

[MRichards99]

Looks good, you just need to fix the merge conflict (looks like a simple one) and then you can merge!