[test] Add isolates testing to CQ

Until now, this test mode was only active in CI. Adding it to CQ should
help catching errors earlier.

TBR=sergiyb@chromium.org

Change-Id: I19a0971ecb318eb5bca3aee435700544d0ad2dcb
Reviewed-on: https://chromium-review.googlesource.com/1131946Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54750}

infra/testing/builders.pyl

@@ -109,6 +109,7 @@
       {'name': 'test262_variants', 'shards': 4},
       {'name': 'test262_variants', 'variant': 'extra', 'shards': 2},
       {'name': 'v8testing'},
+      {'name': 'v8testing', 'suffix': 'isolates', 'test_args': ['--isolates'], 'shards': 2},
       {'name': 'v8testing', 'variant': 'extra'},
     ],
   },