test: make CI faster

[gdalle]

• Bump DI to v0.6.25

Source

• Simplify BatchSizeSettings constructors
• Define an AutoSimpleFiniteDiff backend for centered finite differences

Tests

• Replace AutoForwardDiff with AutoSimpleFiniteDiff in core DI tests
• Remove scenarios involving matrices in benchmarking tests
• Move test categories around and rename Misc into Core
• Make Core DI tests independent from any backend

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.53%. Comparing base (031ea47) to head (40d6d4d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
+ Coverage   97.52%   97.53%   +0.01%     
==========================================
  Files         111      112       +1     
  Lines        5536     5561      +25     
==========================================
+ Hits         5399     5424      +25     
  Misses        137      137              

Flag	Coverage Δ
DI	98.66% <100.00%> (+<0.01%)	⬆️
DIT	95.36% <100.00%> (+<0.01%)	⬆️

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