[test] Bump shards on slow bot

NOTRY=true
TBR=sergiyb@chromium.org

Change-Id: I3751c64f86855d260e4fccd2f86e8958b7a8d9b3
Reviewed-on: https://chromium-review.googlesource.com/c/1307413Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57120}

infra/testing/builders.pyl

@@ -568,7 +568,7 @@
       {'name': 'mozilla', 'variant': 'extra'},
       {'name': 'optimize_for_size'},
       {'name': 'test262_variants', 'shards': 6},
-      {'name': 'test262_variants', 'variant': 'code_serializer', 'shards': 1},
+      {'name': 'test262_variants', 'variant': 'code_serializer', 'shards': 2},
       {'name': 'test262_variants', 'variant': 'extra', 'shards': 2},
       {'name': 'v8testing', 'shards': 3},
       {