tweak TEST_MAPPING for libcore.content.type

CL https://r.android.com/1133257 forgot to add the corresponding
CtsLibcoreTestCases test package to presubmit. This CL adds it.

Test: croot libcore/luni/src/main/java/libcore/content/type && atest
Change-Id: I63e35d492a577775ac0eafd986d91a4eefc11972
diff --git a/luni/src/main/java/libcore/content/type/TEST_MAPPING b/luni/src/main/java/libcore/content/type/TEST_MAPPING
index 8daab75..712f4a2 100644
--- a/luni/src/main/java/libcore/content/type/TEST_MAPPING
+++ b/luni/src/main/java/libcore/content/type/TEST_MAPPING
@@ -1,6 +1,14 @@
 {
   "presubmit": [
     {
+      "name": "CtsLibcoreTestCases",
+      "options": [
+        {
+          "include-filter": "libcore.libcore.content.type"
+        }
+      ]
+    },
+    {
       "name": "CtsMimeMapTestCases"
     }
   ]