Run update_engine_unittests on presubmit

Test: th
Change-Id: I5907c893cc0fa6a42745c9f19e40ddc473372b11
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..93777d4
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+      "name": "update_engine_unittests"
+    }
+  ]
+}