Add trim tests (rebased)

[ChrisRackauckas-Claude]

This is a rebased version of PR #665 onto the latest master.

Summary

Add a trim directory with basic tests for trimming, imitating the code/package loading structure of SciMLBase.

Changes

• Add trim test infrastructure under test/trim/
• Add "trim" test group to CI configuration
• Include JET tests for optimization implementations
• Add trimming tests using juliac for Julia 1.12+
• Test both "clean" (non-trimmable) and "trimmable" implementations
• Add OncePerProcess versions for better trimming support

Test Structure

The tests verify:

1. Clean implementation (non-trimmable) works correctly
2. Trimmable implementation works correctly
3. The trimmable version can be successfully compiled with juliac --trim

Notes

• Trim tests only run on Julia 1.12+ (when trimming was introduced)
• CI excludes trim tests on Julia 1.11 and LTS versions
• Tests use a custom environment in test/trim/ to avoid conflicts

Original PR: #665
Original Author: @RomeoV

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