Add AutomaticTabDiscarding feature to testing configs.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillProfileOrderByFrecency": [
         {
             "group_name": "EnabledLimitTo3",
             "params": {
                 "limit": "3"
             }
         }
     ],
     "AutomaticTabDiscarding": [
         {
+            "enable_features": [
+                "AutomaticTabDiscarding"
+            ],
             "group_name": "Enabled_Once"
         }
     ],
     "CaptivePortalInterstitial": [
         {
             "group_name": "Enabled"
         }
     ],
     "ChromeDashboard": [
         {
@@ skipping 165 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }