Move TestUtils to an extension

[devmotion]

There's a push in the community towards cleaning up dependencies and moving optional features to extension, now that these are supported in Julia >= 1.9. In particular, test utilities are often moved to an extension with a weak dependency on Test (if possible). For instance, this has been done in AbstractFFTs and an open PR to Distributions will hopefully be merged soon.

I suggest we also make Test a weak dependency in DynamicPPL.

[mhauru]

Closing since we'll likely go with a different design for refactoring tests, discussed in TuringLang/Turing.jl#2412.