Flaky test when multithreading

[IanButterworth]

Seen with -t4,4 on #122

Test Failed at D:\a\Distributed.jl\Distributed.jl\test\distributed_exec.jl:252
  Expression: testval == 1
   Evaluated: 0 == 1

https://github.com/JuliaLang/Distributed.jl/actions/runs/12957263140/job/36145255783?pr=122#step:5:93

Seen on

• t4,4
  • linux
  • windows

Passed on a re-run

[IanButterworth]

This was fixed by making this @spawn an @async #122 (comment)