[test] Bump shards after test262 roll

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:7834
Change-Id: Ibd4e57034d493a5cc1a28049e0db0e4bb9c8f996
Reviewed-on: https://chromium-review.googlesource.com/c/1270576Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56462}

infra/testing/builders.pyl

@@ -42,7 +42,7 @@
       {'name': 'benchmarks', 'variant': 'default'},
       {'name': 'v8testing', 'variant': 'default', 'shards': 4},
       {'name': 'mozilla', 'variant': 'default'},
-      {'name': 'test262_variants', 'variant': 'default', 'shards': 6},
+      {'name': 'test262_variants', 'variant': 'default', 'shards': 8},
     ],
   },
   ##############################################################################