[test] Bump shards on slow builder

NOTRY=true
TBR=tmrts@chromium.org

Change-Id: I6ffee170454ca8362591f9f650b3a6ac8bb09552
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776091Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63467}

infra/testing/builders.pyl

@@ -825,7 +825,7 @@
     },
     'tests': [
       {'name': 'mozilla', 'variant': 'default'},
-      {'name': 'test262', 'variant': 'default', 'shards': 4},
+      {'name': 'test262', 'variant': 'default', 'shards': 6},
       {'name': 'v8testing', 'variant': 'default', 'shards': 3},
     ],
   },