[test] Bump shards on slow builder

No-try: true
Bug: v8:5861
Change-Id: I7d09d796788abeced5cae86ff52c052efc0fa456
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2912789
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74718}

infra/testing/builders.pyl

@@ -1023,7 +1023,7 @@
       'os': 'Ubuntu-18.04',
     },
     'tests': [
-      {'name': 'v8testing', 'variant': 'default', 'shards': 4},
+      {'name': 'v8testing', 'variant': 'default', 'shards': 6},
     ],
   },
   'V8 Linux - gc stress': {