test/hil: drop the Windows accommodations, which accommodate nothing

hil_test.py cannot run on Windows and never could: it imports helper.hil_lock,
whose module-level `import fcntl` is POSIX-only, so the harness fails at import
before a line of it executes. Past that it reads /sys/bus/usb, /dev/bus/usb,
/dev/serial/by-id and /proc, kills by process group, and takes flock board
locks -- none of which Windows has.

So the guards were protecting a platform the code cannot reach:

- run_cmd branched three ways on os.name to decide whether to set
  start_new_session and whether to killpg. The non-POSIX arm called p.kill()
  instead, which kills only the direct child -- exactly the semantics the whole
  containment design rejects, since a flasher run through a shell reparents out
  of reach. Dead code that documented the wrong answer.
- hil_test picked multiprocessing's default context on Windows "so it still
  IMPORTS there". It does not import there.
- test_device_audio_test_freertos returned 'skipped' on nt before touching
  ALSA, in a function only ever reached from a worker that cannot start there.
- Seven @unittest.skipIf(os.name == 'nt') decorators across the two suites.
  These were the only ones with a real effect -- the unit tests DO import and
  run on Windows, because they stub pyserial and mostly exercise pure logic --
  but what they buy is a partially-green suite for a harness that cannot run,
  and nothing verifies the set is correct: the hil-test hook only ever runs on
  ubuntu-latest, so a missing guard fails silently until someone tries.

Removing them makes the POSIX assumption single and explicit rather than
scattered and half-honoured. Nothing changes on Linux: every removed branch was
the one already taken there.

Removing the run_cmd guards also removes their `else: p.kill()` arms. Those were
the Windows branches, and p.kill() reaches only the direct child -- a flasher run
through a shell keeps grandchildren it cannot touch, which is the semantics this
containment design rejects. RunCmdCleanupShape pins what is left: both cleanup
paths killpg, no try carries an else whose body would run when the kill
SUCCEEDED, and the BaseException path still re-raises. Structural rather than
behavioural because driving a real SIGINT into a blocked communicate() is
timing-dependent, and what actually breaks this block is an edit that rebinds a
branch -- which is a shape.