[llvm] [lit] Stop holding subprocess objects open in TimeoutHelper (PR #186712)

[Nikita Popov via llvm-commits]

nikic wrote:

For some reason, this seems to have made the "can't start new thread" problem on i686 much worse for us, going from a few dozen Clang tests to 2-3 thousand LLVM tests.

https://github.com/llvm/llvm-project/pull/186712