[test] Bump shards on slow mac builder

NOTRY=true
TBR=tmrts@chromium.org

Change-Id: I2529fde85060571d0fd73e81e384948ef4cbd880
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1678662Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62398}

infra/testing/builders.pyl

@@ -1156,8 +1156,8 @@
     'tests': [
       {'name': 'mozilla'},
       {'name': 'test262', 'shards': 2},
-      {'name': 'v8testing', 'shards': 3},
-      {'name': 'v8testing', 'variant': 'extra'},
+      {'name': 'v8testing', 'shards': 4},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 2},
     ],
   },
   'V8 Mac64 ASAN': {