Mark benchmarks/kraken/audio-dft as slow under slow variants

Bug: v8:12912
Change-Id: Ibfd8d29660ee55fa1d4cd0cac76942826ea541fc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3708852
Auto-Submit: Adam Klein <adamk@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#81202}

test/benchmarks/benchmarks.status

@@ -50,6 +50,7 @@
   # Slow tests.
   'kraken/ai-astar': [PASS, SLOW],
   'kraken/audio-beat-detection': [SLOW],
+  'kraken/audio-dft': [PASS, SLOW],
   'kraken/imaging-darkroom': [PASS, SLOW],
   'kraken/imaging-desaturate': [PASS, SLOW],
   'octane/code-load': [PASS, SLOW],