[devmotion]

Tests have become a bit slow, so this switches test/runtests.jl to ParallelTestRunner.jl, as ChainRules did in JuliaDiff/ChainRules.jl#848. The test files used to run one after another in one process. Now each runs on its own worker process, in parallel.

• find_tests picks up every file under test/ except the helpers utils.jl and derivatives/elementwise_utils.jl. ParallelTestRunner evaluates each file in its own module, so the per-file module ... end wrappers are removed. The nested IsolatedModuleForTestingScoping in ChainRulesTests.jl is kept. Without the wrapper, four loop variables in TrackedTests.jl triggered soft-scope warnings, so they are now declared local.
• The slowest files are split so that no single file sets the wall time. The nested Jacobian tests move from api/JacobianTests.jl to api/NestedJacobianTests.jl. The binary rule tests move from derivatives/ElementwiseTests.jl to derivatives/ElementwiseBinaryTests1.jl and ElementwiseBinaryTests2.jl, which each take every other rule. The shared helpers move to derivatives/elementwise_utils.jl.
• The multithreaded recording test for double free crash with multi-threaded code only when using multiple threads #218 moves from api/GradientTests.jl to test/ThreadsTests.jl, unchanged. ParallelTestRunner's workers are single-threaded, so test_worker gives only this file a dedicated worker with --threads=4. A new @test Threads.nthreads() > 1 makes the test fail if it ever runs on a single-threaded worker, instead of passing without testing anything concurrent.
• JULIA_NUM_THREADS: 4 is removed from CI, since only that test needed it. A side effect is that a local Pkg.test now runs it multithreaded too.
• ParallelTestRunner = "2.0.1" is added to test/Project.toml. v2.0.0 has a bug where find_tests doubles the directory of nested tests, e.g. compat/compat/CompatTests.jl.
• ParallelTestRunner depends on Statistics and loads it on every worker, so StatisticsExt is already loaded when derivatives/LinAlgTests starts. The check that it is not loaded before using Statistics is removed, along with the DOWNGRADE_TEST variable in the Downgrade workflow, which existed only to skip that check.

You can now select tests, e.g. Pkg.test("ReverseDiff"; test_args=["api/GradientTests"]), or list them with test_args=["--list"].

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.27%. Comparing base (ff259e8) to head (26179ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #323   +/-   ##
=======================================
  Coverage   94.27%   94.27%           
=======================================
  Files          19       19           
  Lines        1695     1695           
=======================================
  Hits         1598     1598           
  Misses         97       97           

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