[test] Bump shards on slow bot

TBR=sergiyb@chromium.org
NOTRY=true

Change-Id: I5f7d900a9389872fda69d1ff8d036ee5005f3790
Reviewed-on: https://chromium-review.googlesource.com/c/1290790Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56800}

infra/testing/builders.pyl

@@ -346,7 +346,7 @@
       {'name': 'mozilla', 'shards': 2},
       {'name': 'test262', 'shards': 2},
       {'name': 'v8testing', 'shards': 9},
-      {'name': 'v8testing', 'variant': 'extra', 'shards': 4},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 6},
     ],
   },
   ##############################################################################