test/hil: fail audio test on missing alsa-utils instead of skipping

arecord (alsa-utils) is a documented HIL host requirement, like
mtools/libmtp9/iperf — none of which have a skip-if-missing guard. The
audio test was the exception: it silently returned 'skipped' when arecord
was absent, masking host misconfiguration. The ci.lan rig had been
silently skipping device/audio_test_freertos on every board because
alsa-utils was never installed.

Remove the shutil.which('arecord') guard so a missing package surfaces as
a failure, consistent with the other tool-dependent tests, and drop the
now-unused shutil import. Note in the host-setup comment that these
packages are required (a missing tool fails its test rather than skipping).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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