[test] Mark slow test

NOTRY=true
TBR=mstarzinger@chromium.org,clemensh@chromium.org

Change-Id: I1c4f43b3faa40ab6cc790647aa022890cb418d07
Reviewed-on: https://chromium-review.googlesource.com/607847Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47234}

test/mjsunit/mjsunit.status

@@ -422,6 +422,7 @@
   'regress/regress-91008': [PASS, SLOW],
   'regress/regress-json-stringify-gc': [PASS, SLOW],
   'string-indexof-2': [PASS, SLOW],
+  'wasm/atomics': [PASS, SLOW],
 }],  # 'arch == arm or arch == android_arm'
 
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