Run Background Tasks

Add new syntax to support running background tasks. Stop the background job on exit.

BG "workload_runner" "./workload.sh /mnt/vol1"

Using the name, stop the Background task when needed.

BG_STOP "workload_runner"

Send signal to the background process

BG_SIGNAL "workload_runner", "SIGHUP"

In between other tests, check if background process is running or not when needed.

BG_RUNNING? "workload_runner"

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