Increase Test Coverage

[MRichards99]

This PR will close #217

Description

This branch has some changes I've been working on throughout the week regarding test coverage. Each commit has a distinct test case covered, so referring to the commit messages probably gives the most detail about the changes (without me repeating it here).

One thing worthy of note is that there's not much test coverage increase on query.py. I'm leaving this until I fix the distinct filter stuff on another branch, at which point I'll test query.py more thoroughly on that branch.

Testing Instructions

• Review code
• Check GitHub Actions build
• Review changes to test coverage

Agile Board Tracking

Connect to #217

[codecov]

Codecov Report

Merging #218 (e6aee90) into master (0601fb2) will increase coverage by 1.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   87.95%   89.16%   +1.20%     
==========================================
  Files          29       29              
  Lines        2317     2317              
  Branches      193      193              
==========================================
+ Hits         2038     2066      +28     
+ Misses        242      220      -22     
+ Partials       37       31       -6     

Impacted Files	Coverage Δ
datagateway_api/common/backends.py	100.00% <ø> (+20.00%)	⬆️
datagateway_api/common/icat/filters.py	97.61% <0.00%> (+1.58%)	⬆️
datagateway_api/common/config.py	100.00% <0.00%> (+2.53%)	⬆️
datagateway_api/common/filters.py	100.00% <0.00%> (+4.54%)	⬆️
datagateway_api/common/icat/helpers.py	92.52% <0.00%> (+5.14%)	⬆️
datagateway_api/common/helpers.py	93.84% <0.00%> (+7.69%)	⬆️
datagateway_api/common/exceptions.py	100.00% <0.00%> (+11.76%)	⬆️

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