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

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

Author: BitHighlander

tests/test_msg_session_trust_lifetime.py

@@ -94,17 +94,25 @@ def probe_blob():
 def _emulator_process(port):
     """(pid, exe, cwd) of the process BOUND to udp/port, or None.
 
+    NOTE: subprocess.run(capture_output=/text=) is Python 3.7+. The CI test
+    container runs 3.6, where passing them raises TypeError inside subprocess
+    and this helper dies before any of its own logic runs -- which is why the
+    power-cycle tests FAILED in CI instead of skipping. PIPE plus
+    universal_newlines is the spelling both understand.
+
     Skips this test client's own connected socket, which lsof also reports on
     the same port but as a `local->remote` pair rather than a bare bind.
     """
     try:
         out = subprocess.run(['lsof', '-nP', '-iUDP:%d' % port, '-Fpn'],
-                             capture_output=True, text=True).stdout
-    except FileNotFoundError:
-        raise RuntimeError(
-            "lsof is required to find and restart the emulator for the "
-            "power-cycle tests; install it or run these against a device you "
-            "can power-cycle by hand")
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE, universal_newlines=True).stdout
+    except (FileNotFoundError, OSError):
+        # No lsof: this harness cannot identify, let alone restart, the
+        # emulator process -- the same situation as a remote one. Report "not
+        # found" so _power_cycle() skips with its explanation, rather than
+        # failing a green tree over a missing tool.
+        return None
     pid = None
     for line in out.splitlines():
         if line.startswith('p'):
@@ -114,10 +122,12 @@ def _emulator_process(port):
             if '->' in name or not name.endswith(':%d' % port):
                 continue
             exe = subprocess.run(['ps', '-o', 'comm=', '-p', str(pid)],
-                                 capture_output=True, text=True).stdout.strip()
+                                 stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE, universal_newlines=True).stdout.strip()
             cwd_out = subprocess.run(
                 ['lsof', '-a', '-p', str(pid), '-d', 'cwd', '-Fn'],
-                capture_output=True, text=True).stdout
+                stdout=subprocess.PIPE,
+                             stderr=subprocess.PIPE, universal_newlines=True).stdout
             cwd = None
             for cwd_line in cwd_out.splitlines():
                 if cwd_line.startswith('n'):