Fix the review's findings in the test harness

Headset smoke test: drop the paired key from authorized_keys with a
same-mode copy swapped in, so a failed write can't truncate it; check the
throwaway key with no ssh_config or agent; clean up idempotently (tracked
and leftover titles, their json files, Steam's shortcuts via steamos-delete,
and ~/devkit-utils if it wasn't there before), with a failed cleanup a
failed step; count a launch only with fresh evidence (the process, Steam's
log, or the known missing-runtime line), matching with [d]evkit-game so
pgrep doesn't find its own shell. The test programs sleep 10 s.

Fake Frame: log a launch before its reaper can look for it. e2e: kill a
pairing client's process group when a test ends; accept an aarch64 program
running under QEMU on x86 hosts.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/e2e/test_pairing.py

@@ -2,6 +2,7 @@
 steamos-devkit-service on the fake Frame, and its password fallback."""
 import json
 import os
+import signal
 import stat
 import subprocess
 import sys
@@ -28,9 +29,20 @@ class Pairing(harness.FrameTestCase):
                 f.write(f'#!/bin/sh\necho {askpass}\n')
             os.chmod(script, stat.S_IRWXU)
             env.update(SSH_ASKPASS=script, SSH_ASKPASS_REQUIRE='force')
-        return subprocess.Popen([sys.executable, str(ROOT / 'ui' / 'frame_connect.py'), FAKE_HOST],
+        proc = subprocess.Popen([sys.executable, str(ROOT / 'ui' / 'frame_connect.py'), FAKE_HOST],
                                 stdin=subprocess.DEVNULL, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
                                 text=True, env=env, start_new_session=True)
+        self.addCleanup(self.stop, proc)    # a failed test mustn't leave it pairing into the next one
+        return proc
+
+    @staticmethod
+    def stop(proc):
+        if proc.poll() is None:
+            try:
+                os.killpg(proc.pid, signal.SIGKILL)     # frame_connect.py and its ssh children
+            except OSError:
+                pass
+        proc.communicate()
 
     def finish(self, proc, timeout=120):
         out, _ = proc.communicate(timeout=timeout)