Canonicalize tests to @safetestset for module isolation

[ChrisRackauckas-Claude]

What

Converts the independent top-level test units in test/runtests.jl from plain @testset blocks into @safetestset blocks, so each runs in its own freshly-generated module. This matches the canonical OrdinaryDiffEq test structure and gives per-unit isolation plus world-age safety.

Changes

• The five units (Fast log2, Fast pow, Fast pow - Enzyme forward rule, Fast pow - Enzyme reverse rule, Fast pow - Other AD Engines) become @safetestset blocks.
• Each unit is now self-contained: the using/import lines its body relies on are copied into the block (previously these were shared module-level imports at the top of runtests.jl).
• The file-scope helper mooncake_derivative moves inside the one unit that uses it (Fast pow - Other AD Engines), since each @safetestset is its own module.
• Inner @testset for ... loops (Enzyme forward/reverse rules) stay as plain @testset — the unit-level @safetestset already isolates them.
• Adds SafeTestsets to the test deps ([compat], [extras], [targets].test).

No test logic, assertions, or test counts were changed; the GROUP/run structure is untouched (there is no GROUP ladder here).

Verification

Verified locally with julia +1.11, Pkg.test() — all units pass with identical counts:

Fast log2                      | 1200  1200
Fast pow                       |    5     5
Fast pow - Enzyme forward rule |   52    52
Fast pow - Enzyme reverse rule |   36    36
Fast pow - Other AD Engines    |    4     4
     Testing FastPower tests passed

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Superseded by the v1.2 folder conversion on sciml-testing-rollout (#54).