Convert hfi_calc bdd tests

[dgboss]

• Convert hfi_calc fire_centres route tests from bdd to vanilla pytest
• closes Dev Task: Replace test_hfi_planning_areas.feature with vanilla pytest tests #4025

Test Links:

Landing Page
MoreCast
Percentile Calculator
C-Haines
FireBat
FireBat bookmark
Auto Spatial Advisory (ASA)
HFI Calculator

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.43%. Comparing base (8d58b59) to head (69387f0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4128      +/-   ##
==========================================
- Coverage   80.94%   80.43%   -0.52%     
==========================================
  Files         309      309              
  Lines       11876    11876              
  Branches      537      537              
==========================================
- Hits         9613     9552      -61     
+ Misses       2145     2138       -7     
- Partials      118      186      +68     

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