[test] Bump shards on slow builder

NOTRY=true
TBR=leszeks@chromium.org

Change-Id: I5abb432e42168484aabf04600e8e2cf6e3511630
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2388105Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69654}

infra/testing/builders.pyl

@@ -1149,7 +1149,7 @@
       {
         'name': 'v8testing',
         'test_args': ['--extra-flags=--stress-incremental-marking'],
-        'shards': 4,
+        'shards': 6,
       },
     ],
   },