DocTestTeardown for cleaning-up the DocTestSetup state (or similar feature)

[maleadt]

I'm using DocTestSetup in LLVM.jl to set-up the resource context needed to execute doctests in:

```@meta
DocTestSetup = quote
    using LLVM
    ctx = Context()
end
```

However, that context is supposed to get cleaned up (not only to avoid memory leaks, but also to detect situations where accidentally using resources from a different context). It would be nice if there were a DocTestTeardown where I could put the clean-up:

```@meta
DocTestTeardown = quote
    dispose(ctx)
end
```

Right now, I'm using the following workaround:

```@meta
DocTestSetup = quote
    using LLVM

    # XXX; clean-up previous contexts
    while context(; throw_error=false) !== nothing
        dispose(context())
    end

    ctx = Context()
end
```

Also requested on Discourse, https://discourse.julialang.org/t/is-it-possible-to-run-a-cleanup-code-after-a-jldoctest-block-has-been-evaluated/44999, by @jishnub

[lgoettgens]

I am working on this and #2058. I'll open a draft PR once there is something to show