Add known values of 2025 policy parameters

[martinholmer]

Fixes #2829.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (aaa4eb9) to head (46e1e8c).
Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2837   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2617     2617           
=======================================
  Hits         2602     2602           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <100.00%> (ø)

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Files with missing lines	Coverage Δ
taxcalc/policy.py	100.00% <100.00%> (ø)