Add mainline-postsubmit to CaptivePortalLogin

The tests will then be added to mainline-presubmit after fixing any failure
seen in mainline-postsubmit.

Change-Id: Ie28b27110b687b6b896a5dbe29f623d02d9bc3f2
Test: Treehugger needs to verify test mapping
diff --git a/TEST_MAPPING b/TEST_MAPPING
index ebea16d..c2cd949 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -3,5 +3,10 @@
     {
       "name": "CaptivePortalLoginTests"
     }
+  ],
+  "mainline-postsubmit": [
+    {
+      "name": "CaptivePortalLoginTests[CaptivePortalLoginGoogle.apk+NetworkStackGoogle.apk+com.google.android.resolv.apex+com.google.android.tethering.apex]"
+    }
   ]
 }