Increase Test Coverage

[MRichards99]

This PR will close #250

Description

After learning how to do test mocking, just thought I'd try and increase test coverage where there's easy wins to do so. Like I've done before, I've kind've left the DB backend coverage alone as the backend specific tests might be able to be combined through #213 in the future.

Testing Instructions

No functionality change, just make sure you're happy with the new tests :)

• 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 there an underlying issue with the changes made?
• Review changes to test coverage

Agile Board Tracking

Connect to #250

[codecov]

Codecov Report

Merging #251 (9e0977d) into master (1130f4e) will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
+ Coverage   90.55%   91.10%   +0.55%     
==========================================
  Files          31       31              
  Lines        2361     2361              
  Branches      205      205              
==========================================
+ Hits         2138     2151      +13     
+ Misses        194      186       -8     
+ Partials       29       24       -5     

Impacted Files	Coverage Δ
datagateway_api/common/icat/filters.py	99.33% <0.00%> (+1.32%)	⬆️
datagateway_api/common/icat/helpers.py	94.57% <0.00%> (+1.80%)	⬆️
datagateway_api/common/query_filter_factory.py	100.00% <0.00%> (+6.45%)	⬆️
datagateway_api/common/icat/icat_client_pool.py	100.00% <0.00%> (+7.69%)	⬆️
...tagateway_api/src/resources/entities/entity_map.py	95.65% <0.00%> (+8.69%)	⬆️

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