[test] Bump shards on slow arm64 trybot

NOTRY=true
TBR=jgruber@chromium.org

Change-Id: Iac18f84235fe2013c3ffb3af56f95827082fa73a
Reviewed-on: https://chromium-review.googlesource.com/1070201Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53304}

infra/testing/builders.pyl

@@ -248,7 +248,7 @@
       {'name': 'mjsunit_sp_frame_access'},
       {'name': 'mozilla'},
       {'name': 'test262'},
-      {'name': 'v8testing', 'shards': 7},
+      {'name': 'v8testing', 'shards': 9},
       {'name': 'v8testing', 'variant': 'extra', 'shards': 3},
     ],
   },