Enable ART's target gtests in postsubmit.

Test: atest :postsubmit
Change-Id: I0a13a93e4b78403fdfe07e95e90473e2210b5ccb
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 0c53164..58295a4 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -3,5 +3,10 @@
     {
       "name": "CtsJdwpTestCases"
     }
+  ],
+  "postsubmit": [
+    {
+      "name": "ArtGtestsTarget"
+    }
   ]
 }