Add rust_test and update TEST_MAPPING

Test: atest -c --include-subdirs external/rust/crates
Change-Id: If1c7cb1933ca92800e7433ebbff05a8f37ff67ed
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..ee02dc9
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,9 @@
+// Generated by cargo2android.py for tests in Android.bp
+{
+  "presubmit": [
+    {
+      "name": "protobuf-codegen_host_test_src_lib",
+      "host": true
+    }
+  ]
+}