[test] Bump shards on slow bot

TBR=sergiyb@chromium.org,tmrts@chromium.org
NOTRY=true

Change-Id: I4fe37f8e38922e2a7d08a6b28191eff50778e0b2
Reviewed-on: https://chromium-review.googlesource.com/c/1450121Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59306}

infra/testing/builders.pyl

@@ -689,7 +689,7 @@
     },
     'tests': [
       {'name': 'mozilla', 'variant': 'default'},
-      {'name': 'test262', 'variant': 'default', 'shards': 3},
+      {'name': 'test262', 'variant': 'default', 'shards': 4},
       {'name': 'v8testing', 'variant': 'default', 'shards': 3},
     ],
   },