Speed up CI

[gdalle]

DI CI and tests

• Set fail-fast to true so that any failing matrix job interrupts the whole matrix
• Remove lts and pre matrix entries before launching the jobs when on draft PR
• Put all test files on the same nesting level to simplify runtests.jl
• Group some related tests:
  • DI internals
  • ChainRules backends (ChainRulesCore + Diffractor)
  • Symbolic backends (FastDifferentiation + Symbolics)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 28.57143% with 10 lines in your changes missing coverage. Please review.

Project coverage is 98.60%. Comparing base (0f44787) to head (71f274b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
DifferentiationInterface/test/runtests.jl	28.57%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #522      +/-   ##
==========================================
- Coverage   98.71%   98.60%   -0.12%     
==========================================
  Files         106      106              
  Lines        4585     4578       -7     
==========================================
- Hits         4526     4514      -12     
- Misses         59       64       +5     

Flag	Coverage Δ
DI	98.71% <28.57%> (-0.17%)	⬇️
DIT	98.35% <ø> (ø)

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