Give the audio output sink test the standard header

docs/testing.md asks every shell test to start with set -euo pipefail and source base-test.sh through BASH_SOURCE; without it a failed mktemp leaves test_home empty and reset_scenario goes on to rm -rf /data.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/shell.d/audio-output-sink-test.sh

@@ -1,6 +1,8 @@
 #!/bin/bash
 
-source "$(dirname "$0")/base-test.sh"
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
 
 test_home=$(mktemp -d)
 test_bin=$(mktemp -d)