[test] Bump test262 shards on mac

NOTRY=true
TBR=sergiyb@chromium.org

Bug: v8:8552
Change-Id: I096146072915f2d5baf5d4469421696cf31d72c2
Reviewed-on: https://chromium-review.googlesource.com/c/1363136Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58042}

infra/testing/builders.pyl

@@ -491,7 +491,7 @@
     },
     'tests': [
       {'name': 'mozilla'},
-      {'name': 'test262'},
+      {'name': 'test262', 'shards': 2},
       {'name': 'v8testing', 'shards': 2},
       {'name': 'v8testing', 'variant': 'extra'},
     ],
@@ -924,7 +924,7 @@
     'tests': [
       {'name': 'mozilla'},
       {'name': 'test262'},
-      {'name': 'v8testing'},
+      {'name': 'v8testing', 'shards': 2},
       {'name': 'v8testing', 'variant': 'extra'},
     ],
   },
@@ -935,7 +935,7 @@
     },
     'tests': [
       {'name': 'mozilla'},
-      {'name': 'test262'},
+      {'name': 'test262', 'shards': 2},
       {'name': 'v8testing', 'shards': 3},
       {'name': 'v8testing', 'variant': 'extra'},
     ],