[test] Mark slow test

No-Try: true
Bug: v8:10952
Change-Id: Iee858419b160a354a4df61f51b18782fcbc4a521
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424155
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70089}

test/mjsunit/mjsunit.status

@@ -155,6 +155,7 @@
   'wasm/asm-wasm-f32': [PASS, SLOW],
   'wasm/asm-wasm-f64': [PASS, SLOW],
   'wasm/embenchen/*': [PASS, SLOW],
+  'wasm/futex': [PASS, SLOW],
   'wasm/grow-memory': [PASS, SLOW],
   'wasm/unreachable-validation': [PASS, SLOW],
   'wasm/atomics-stress': [PASS, SLOW, NO_VARIANTS, ['mode != release or dcheck_always_on', SKIP], ['tsan', SKIP]],