Background process can hold onto streams, causing jobs to not finish

[vtbassmatt]

On Linux, the test agents hang indefinitely under the following conditions:

• A main process is launched by the agent
• Main process launches a new process which inherits the handles of the main process
• Main process exits before the child process

See https://github.com/natemcmaster/repro-vsts-stdout-blocking

We talked about addressing this by watching a couple of Node events and, if some threshold of time elapses between them, letting the user know what's happening. Optionally we could fulfill the promise so Node forcibly returns.

[nigurr]

@ericsciple We are also facing the same issue in test execution task. If test code leaves any stale process (typically chrome driver), we end up hitting task timeout in the end even though test execution is completed.

[ericsciple]

fixed with commit 800dadc (forgot to close this issue)