[test] Bump shards on slow builder

NOTRY=true
TBR=mslekova@chromium.org

Change-Id: I3793426aaebd400ac93015b85ef11db5aecef52f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1795357Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63649}

infra/testing/builders.pyl

@@ -227,7 +227,7 @@
       {'name': 'mozilla'},
       {'name': 'test262'},
       {'name': 'v8testing', 'shards': 7},
-      {'name': 'v8testing', 'variant': 'extra', 'shards': 3},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 6},
       {'name': 'v8testing', 'variant': 'trusted', 'shards': 2},
     ],
   },
@@ -1394,7 +1394,7 @@
       {'name': 'mozilla', 'shards': 4},
       {'name': 'test262'},
       {'name': 'v8testing', 'shards': 8},
-      {'name': 'v8testing', 'variant': 'extra', 'shards': 4},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 6},
       {'name': 'v8testing', 'variant': 'trusted', 'shards': 2},
       # Armv8-a.
       {