Process termination monitoring implementation on Linux conflicts with processes spawned by other means

[jakepetroules]

The approach used by Subprocess to monitor subprocess termination on Linux is fundamentally flawed as it calls waitid with P_ALL, and WEXITED without WNOWAIT, which will end up reaping pids that were spawned outside the Subprocess library.

We need to use an implementation more like https://github.com/swiftlang/swift-testing/blob/main/Sources/Testing/ExitTests/WaitFor.swift, which doesn't suffer from this issue.