Hang if subprocess spawns dependent subprocess

[harrisonliew]

In this scenario, Hammer will hang:

1. Hammer spawns a subprocess running a tool (e.g. a fill deck)
2. The tool's script spawns some other process that waits for the tool to finish before running (e.g. merging fill with input database, like #287)
3. As a result, there is a bidirectional wait dependency and so the tool never exits.

It appears the subprocess module can't support this scenario? At the moment, these kinds of scripts need to be run outside of Hammer.

[harrisonliew]

Possibly related: when VCS runs the compiled simulator, that output is not captured by the Hammer logs either.