Run CtsLibcoreOjTestCases as postsubmit

Plan to run this as presubmit once the postsubmit runs are stablized.

Bug: 300434916
Test: atest --test-mapping libcore/
Change-Id: I14076bd7b0349a3674b0842d4b8b99556ff14440
diff --git a/TEST_MAPPING b/TEST_MAPPING
index c943484..98fc58a 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -92,6 +92,16 @@
       ]
     }
   ],
+  "postsubmit": [
+    {
+      "name": "CtsLibcoreOjTestCases",
+      "options": [
+        {
+          "instrumentation-arg": "core-test-mode:=presubmit"
+        }
+      ]
+    }
+  ],
   "kernel-presubmit": [
     {
       "name": "CtsLibcoreTestCases",