[test] Bump shards on slow bots

NOTRY=true

TBR=sergiyb@chromium.org

Change-Id: Iaff2d456af6e6494c725efa60cbdc9895201bef5
Reviewed-on: https://chromium-review.googlesource.com/c/1466962Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59537}

infra/testing/builders.pyl

@@ -43,7 +43,7 @@
       {'name': 'v8testing', 'variant': 'default', 'shards': 4},
       {'name': 'v8testing', 'variant': 'trusted', 'shards': 4},
       {'name': 'mozilla', 'variant': 'default'},
-      {'name': 'test262_variants', 'variant': 'default', 'shards': 6},
+      {'name': 'test262_variants', 'variant': 'default', 'shards': 7},
     ],
   },
   ##############################################################################
@@ -1054,7 +1054,7 @@
     },
     'tests': [
       {'name': 'mozilla', 'variant': 'default'},
-      {'name': 'test262', 'variant': 'default', 'shards': 5},
+      {'name': 'test262', 'variant': 'default', 'shards': 6},
       {'name': 'v8testing', 'variant': 'default', 'shards': 3},
       {'name': 'v8testing', 'variant': 'trusted', 'shards': 3},
     ],