headless: Don't run tests during the build

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
@@ skipping 9693 matching lines @@
           "can_use_on_swarming_builders": false
         },
         "test": "headless_browsertests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "headless_unittests"
       }
+    ],
+    "scripts": [
+      {
+        "name": "headless_python_unittests",
+        "script": "headless_python_unittests.py"
+      }
     ]
   },
   "LTO Linux": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
@@ skipping 2745 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "views_unittests"
       }
     ]
   }
 }