tests: fix memres start hanging in subprocess.run

memres start spawns background processes (QEMU VM, idle watchdog)
that persist after the vrunner script exits. When invoked via
subprocess.run(capture_output=True), these background processes
inherit the pipe file descriptors, preventing communicate() from
returning until all pipe holders exit — which can be 30+ minutes
(the idle timeout).

Fix by using Popen with:
- stdin=subprocess.DEVNULL (no inherited stdin pipe)
- file-based stdout (no pipe FDs to inherit)
- start_new_session=True (new process group, so wait() only
  waits for the parent script, not the background children)

This matches the behavior when running from a shell, where the
daemon processes are fully detached from the caller's FD table.

Applied to both VdkrRunner and VpdmnRunner memres_start methods.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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