Pkg.test can't access docstring values via @doc on recent Julia nightly / 1.11

[ztangent]

Discovered this issue when our CI workflow runs for Gen.jl started failing on Julia nightly.

The following code works fine if you run it in the REPL:

using Test

"my function"
function foo()
    return 0
end

io = IOBuffer()
print(io, @doc foo)
@test String(take!(io)) == "my function\n"

However, once you put it in a runtests.jl file of a package, then run Pkg.test on that package, you get the following error:]

     Testing Running tests...
Test Failed at [...]\TestDocstring.jl\test\runtests.jl:10
  Expression: String(take!(io)) == "my function\n"
   Evaluated: "nothing" == "my function\n"

It looks like for some reason @__doc__ foo is returning "nothing" instead of the docstring, and that this only happens within Pkg.test.

Not sure if this is an issue with something in Julia or something in Pkg.jl, but figured I'd file the issue here.

Here's the version info (but the failure also occurs on 1.11.0-DEV.637 and 1.11.0-DEV.636, i.e. the previous two commits):

Julia Version 1.11.0-DEV.638
Commit 3711749292 (2023-10-11 14:41 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: 8 × Intel(R) Core(TM) i7-8665U CPU @ 1.90GHz
  WORD_SIZE: 64
  LLVM: libLLVM-15.0.7 (ORCJIT, skylake)
  Threads: 1 on 8 virtual cores

[KristofferC]

Dup of JuliaLang/julia#52986