Organise unit tests to use pytest fixtures (`test_config.py` implementation)

[SeanBryan51]

This change was very painful. Hopefully once #143 is implemented we can remove these tests completely!

[codecov[bot]]

Codecov Report

Merging #177 (226b821) into 163-tidy-up-organisation-of-unit-tests (b3a0fac) will increase coverage by 15.17%. The diff coverage is 100.00%.

@@                             Coverage Diff                             @@
##           163-tidy-up-organisation-of-unit-tests     #177       +/-   ##
===========================================================================
+ Coverage                                   60.45%   75.63%   +15.17%     
===========================================================================
  Files                                          25       25               
  Lines                                        1659     1658        -1     
===========================================================================
+ Hits                                         1003     1254      +251     
+ Misses                                        656      404      -252     

Files	Coverage Δ
tests/test_config.py	100.00% <100.00%> (+98.24%)	:arrow_up:

... and 1 file with indirect coverage changes

[bschroeter]

This change was very painful. Hopefully once #143 is implemented we can remove these tests completely!

Indeed!