[test] Bump shards on slow branch builder

NOTRY=true
TBR=tmrts@chromium.org

Change-Id: I44bff77691130b39b4033127dc837ef9d27a740c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1870232
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64400}

infra/testing/builders.pyl

@@ -1780,9 +1780,9 @@
       'os': 'Ubuntu-16.04',
     },
     'tests': [
-      {'name': 'mozilla'},
-      {'name': 'test262', 'variant': 'default'},
-      {'name': 'v8testing', 'shards': 8},
+      {'name': 'mozilla', 'shards': 2},
+      {'name': 'test262', 'variant': 'default', 'shards': 2},
+      {'name': 'v8testing', 'shards': 10},
     ],
   },
   'V8 arm - sim - stable branch': {
@@ -1800,9 +1800,9 @@
       'os': 'Ubuntu-16.04',
     },
     'tests': [
-      {'name': 'mozilla'},
-      {'name': 'test262', 'variant': 'default'},
-      {'name': 'v8testing', 'shards': 8},
+      {'name': 'mozilla', 'shards': 2},
+      {'name': 'test262', 'variant': 'default', 'shards': 2},
+      {'name': 'v8testing', 'shards': 10},
     ],
   },
   'V8 mips64el - sim - beta branch': {