[test] Bump shards on slow bot

NOTRY=true
TBR=sergiyb@chromium.org

Change-Id: Ie45a93dc851f0bc12800cbd91fe324c854b1dcb6
Reviewed-on: https://chromium-review.googlesource.com/c/1335448Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57487}

infra/testing/builders.pyl

@@ -679,7 +679,7 @@
     },
     'tests': [
       {'name': 'mozilla', 'variant': 'default'},
-      {'name': 'test262', 'variant': 'default', 'shards': 2},
+      {'name': 'test262', 'variant': 'default', 'shards': 3},
       {'name': 'v8testing', 'variant': 'default', 'shards': 3},
     ],
   },