Move most of TestUtils into TestExt

[penelopeysm]

This PR moves the implementations of src/test_utils/context.jl, src/test_utils/sampler.jl, and src/test_utils/varinfo.jl into a new DynamicPPLTestExt extension.

Benefits:

• This functionality is only used for testing and thus shouldn't really be loaded when the user doesn't want to test. In this sense it is the 'correct' thing to do
• The benefit of that is, as with any other extension, it reduces load times for users who don't need the functionality

Negatives:

• Due to the way extensions work in Julia there is no way to namespace functions in an extension. The workaround for this is to define a module plus an empty function inside src/test_utils.jl and then extend this inside TestExt with the real implementation.
• I don't think this works for types, unfortunately, so I had to leave the definition of one struct inside src/test_utils.jl.

Possible ways forward

1. Accept this ugly situation and merge this PR.
2. Do nothing, i.e. close this PR and keep everything inside src.
3. Move everything to a different package, which can then be declared as a test dep.

My hot take is that I'd prefer (3). Well .... kind of, at least. If we have assorted stuff that builds on DPPL, e.g. benchmarking, I still kind of feel that it would be better to have it in a separate package.

Maybe one possibility is to have the different packages in the same repo so that CI + development doesn't fall out of sync? I recognise this would require a fair bit of reorganisation.

Closes #550

[mhauru]

1. sounds quite confusing to me, although you have documented it. If we go with it, I would add extra documentation explaining why this is needed. But I wonder if the loss of code readability is worth the load time gain. If I was implementing this I would probably go for 2. I fear that 3. would make edits hard and annoying, because of splitting of closely related code between repos. For instance, making a small edit to the test suite that then depends on you making an edit to a separate package sounds like an annoying dev experience.

[penelopeysm]

CI has a bunch of UndefVarErrors probably because of moving things around. I won't bother fixing them unless we decide to go ahead with this 😅