Move CaptivePortalLoginTests to mainline-presubmit

The tests have been stable in mainline-postsubmit for a while.

Change-Id: Iaaff06b9d216493055f38bbe91406306d3ebbaed
Test: TH needs to test TEST_MAPPING change
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 322c02b..cd9448b 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -4,7 +4,7 @@
       "name": "CaptivePortalLoginTests"
     }
   ],
-  "mainline-postsubmit": [
+  "mainline-presubmit": [
     {
       "name": "CaptivePortalLoginTests[CaptivePortalLoginGoogle.apk+NetworkStackGoogle.apk+com.google.android.resolv.apex+com.google.android.tethering.apex]"
     }