test: more robust URL coverage; improve badly formed test

[jennifer-richards]

Main work here is to use Django's resolve() rather than applying our own regex matching to our url coverage test. This ensures that we're accounting for coverage that agrees with what Django is doing. Motivated because path URL patterns handle end-of-string anchors in a way that the old regex code does not account for, so overlapping URLs may be accounted incorrectly. That is, two patterns

• ^api/purple/rfc/publish/
• ^api/purple/rfc/publish/files/

may both match the URL api/purple/rfc/publish/files/ because the regexes don't have a trailing $. The path gearing applies the $, but our code did not.

Also fixes a random other test that was passing, but using unrealistic data.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.41%. Comparing base (31b3b82) to head (e119793).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10270   +/-   ##
=======================================
  Coverage   88.40%   88.41%           
=======================================
  Files         324      324           
  Lines       43344    43344           
=======================================
+ Hits        38318    38322    +4     
+ Misses       5026     5022    -4     

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