Add rust_test and update TEST_MAPPING

Test: atest -c --include-subdirs external/rust/crates
Change-Id: Ica179004c4c689a8c619824db4bbe24d1fff285f
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..df5b82f
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,12 @@
+// Generated by cargo2android.py for tests in Android.bp
+{
+  "presubmit": [
+    {
+      "name": "unicode-width_host_test_src_lib",
+      "host": true
+    },
+    {
+      "name": "unicode-width_device_test_src_lib"
+    }
+  ]
+}