[test] Add test configs for experimental builder

TBR=sergiyb@chromium.org
NOTRY=true

Bug: chromium:890222
Change-Id: I5a48dca2a4c1dec61d21d068eee4b49d138a5c4b
Reviewed-on: https://chromium-review.googlesource.com/1256244Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56330}

infra/testing/builders.pyl

@@ -278,6 +278,17 @@
       {'name': 'v8testing', 'variant': 'slow_path'},
     ],
   },
+  # TODO(machenbach): Experimental builder with incomplete configs. Should be
+  # similar to v8_linux64_rel_ng_triggered after testing.
+  'v8_linux64_rel_xg': {
+    'swarming_dimensions' : {
+      'cpu': 'x86-64-avx2',
+    },
+    'tests': [
+      {'name': 'v8initializers'},
+      {'name': 'v8testing', 'shards': 2},
+    ],
+  },
   'v8_linux64_tsan_rel': {
     'tests': [
       {'name': 'benchmarks'},