[test] Run test262 on mac on more shards

The test runner is stuck on start-up otherwise.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:8552
Change-Id: Ief2632ce168f83ae33bc9ae1f7edee152505cae9
Reviewed-on: https://chromium-review.googlesource.com/c/1437276Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59107}

infra/testing/builders.pyl

@@ -490,7 +490,7 @@
     },
     'tests': [
       {'name': 'mozilla'},
-      {'name': 'test262'},
+      {'name': 'test262', 'shards': 2},
       {'name': 'v8testing', 'shards': 3},
       {'name': 'v8testing', 'variant': 'extra', 'shards': 2},
     ],
@@ -946,7 +946,7 @@
     },
     'tests': [
       {'name': 'mozilla'},
-      {'name': 'test262'},
+      {'name': 'test262', 'shards': 2},
       {'name': 'v8testing', 'shards': 2},
       {'name': 'v8testing', 'variant': 'extra'},
     ],