fix(tests): power-cycle helper must run on Python 3.6, which CI uses

[BitHighlander]

The two power-cycle lifetime tests kept failing in CI after being taught to skip — and the skip was never the problem:

_emulator_process() -> subprocess.run(..., capture_output=True, text=True)
TypeError: __init__() got an unexpected keyword argument 'capture_output'

capture_output= and text= are Python 3.7+. The CI python-keepkey container runs 3.6, so the helper raised inside subprocess before any of its own logic — including the skip added last commit — could run. Locally it passed because this machine runs 3.10.

Replaced all three uses with stdout=/stderr=PIPE plus universal_newlines, which 3.6 and 3.10 both understand, and recorded why in the docstring so nobody "modernises" it back.

Also: a missing lsof now returns None instead of raising. Same situation as a remote emulator — the harness cannot identify the process, let alone restart it — so it belongs on the skip path, not the failure path. A green tree should not go red because a container lacks a tool.

Worth stating for the next person: this failure was invisible locally in every run, and the previous fix looked correct precisely because it was tested on the wrong interpreter. The environment is part of the test.

Local (3.10, harness owns the emulator): 6/6, both power-cycle tests executing rather than skipping.