[test] Mark slow test

The test mjsunit/wasm/shared-memory-worker-gc is too slow on the gcov
bot.

No-Try: true
Bug: v8:13005
Change-Id: Idac2a6df836c981195d61f9c2737c06d548edb28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3751204
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81587}

test/mjsunit/mjsunit.status

@@ -83,6 +83,7 @@
 
   # BUG(v8:9506): slow tests.
   'wasm/shared-memory-worker-explicit-gc-stress': [PASS, SLOW],
+  'wasm/shared-memory-worker-gc': [PASS, SLOW],
   'wasm/shared-memory-worker-gc-stress': [PASS, SLOW],
 
   # https://crbug.com/1129854