Move ART gtests to the `presubmit` test group in `TEST_MAPPING`.

Test: atest --test-mapping art:presubmit
Bug: 152379281
Change-Id: Ieae5f3d8ee69c71bcdac4a7e006d82e193a12715
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 1622c5b..132cdb2 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -6,13 +6,13 @@
     },
     {
       "name": "BootImageProfileTest"
-    }
-  ],
-  "postsubmit": [
+    },
     // ART gtests.
     {
       "name": "ArtGtestsTarget"
-    },
+    }
+  ],
+  "postsubmit": [
     // ART run-tests.
     {
       "name": "art-run-test-001-HelloWorld"