Convert settings views' tests to pytest syntax

[rebeccacremona]

This PR converts all the tests for the settings views, extracted in #3607, to pytest syntax, and switches them to use pytest fixtures instead of JSON fixtures + mocks.

I don't love some of these tests, but I resisted the urge to change them too much: not the goal of this exercise 🤣.

As before, I think only a lightweight review is in order, and let it be on my head if I messed things up in the conversion: not worth anybody's time to study this.

But, a quick buzz through for code style would be appreciated. Do you see anything that offends sensibilities?

[cmsetzer]

Looks great! Thanks so much for this, I'm sure it was a slog but this will set the table for lots of further test improvements.

I included a few optional stylistic suggestions; feel free to adopt or ignore them at your discretion.

[codecov]

Codecov Report

Attention: Patch coverage is 99.18033% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.73%. Comparing base (c72853c) to head (f401f93).
Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
perma_web/conftest.py	99.18%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3613      +/-   ##
===========================================
+ Coverage    69.19%   69.73%   +0.54%     
===========================================
  Files           53       54       +1     
  Lines         7193     7326     +133     
===========================================
+ Hits          4977     5109     +132     
- Misses        2216     2217       +1     

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