Canonicalize tests to @safetestset for module isolation

[ChrisRackauckas-Claude]

Summary

Canonicalizes test/runtests.jl so each independent test unit runs in its own module via @safetestset, matching the canonical OrdinaryDiffEq structure (isolation between tests + world-age safety).

Classification

PLAIN-TESTSET: the suite was a single outer @testset "DifferentialEquations" wrapping 8 independent inner plain @testset units, all sharing only the top-level using DifferentialEquations, Test, SafeTestsets.

What changed

• Each inner @testset "X" begin ... end -> @safetestset "X" begin ... end.
• Each @safetestset body is made self-contained with its own using DifferentialEquations, Test.
• The outer @testset "DifferentialEquations" wrapper and the @time begin ... end block are kept, so the grouped test summary is preserved (@safetestset nests fine inside @testset).

What did NOT change

• No test logic or assertions changed. The same 17 assertions run.
• These bodies use no package macros and only resolve package symbols (ODEProblem, Tsit5, solve, ...) at runtime, so an inline @safetestset body with using DifferentialEquations, Test is sufficient (no include() indirection needed).
• SafeTestsets was already in [extras] + [targets].test, so Project.toml is unchanged.

Verification

Ran locally with julia +1.11 Pkg.test():

Test Summary:         | Pass  Total   Time
DifferentialEquations |   17     17  22.3s
     Testing DifferentialEquations tests passed

Ignore until reviewed by @ChrisRackauckas.

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Superseded by the SciMLTesting v1.2 PR #1145 (grouped-tests + run_tests + @safetestset). Closing this canonicalize-only PR.