Put testing utilities in extension

[gdalle]

• Define Scenario struct and empty test functions in the DifferentiationInterface.DifferentiationTest submodule
• Move the rest of the code from test/utils.jl and test/scenarios.jl to the extension DifferentiationInterfaceTestExt, triggered by ForwardDiff, JET, Random, Test
• Reduce the depth of nested loops by using @testset for
• Add dummy zero backends to test type stability of the fallback structure
• Remove allocation checks cause they would need to be done with BenchmarkTools to be accurate
• Add nice hints for MethodErrors to load extensions with https://docs.julialang.org/en/v1/base/base/#Base.Experimental.register_error_hint

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.69202% with 35 lines in your changes are missing coverage. Please review.

Project coverage is 92.60%. Comparing base (b6be4a5) to head (3236b57).

Files	Patch %	Lines
ext/DifferentiationInterfaceTestExt/test.jl	82.32%	35 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   95.64%   92.60%   -3.04%     
==========================================
  Files          22       26       +4     
  Lines         390      649     +259     
==========================================
+ Hits          373      601     +228     
- Misses         17       48      +31     

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[gdalle]

Missing code cov is due to @testsets not being counted, unsure why