[test] Bump shards on slow bot

NOTRY=true
TBR=sergiyb@chromium.org

Change-Id: I61597a265e5d98479cb88f5c8e01fed317044dd6
Reviewed-on: https://chromium-review.googlesource.com/c/1350998Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57856}

infra/testing/builders.pyl

@@ -636,7 +636,7 @@
       'priority': 35,
     },
     'tests': [
-      {'name': 'v8testing', 'variant': 'default', 'shards': 2},
+      {'name': 'v8testing', 'variant': 'default', 'shards': 3},
     ],
   },
   'V8 Linux - gc stress': {