Update TEST_MAPPING

Test: cd external/rust/crates && atest --host -c
Change-Id: Ia96b1a82efe51d47dbfee6b3216b1920ef3638bf
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..5cbb502
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,8 @@
+// Generated by update_crate_tests.py for tests that depend on this crate.
+{
+  "imports": [
+    {
+      "path": "external/rust/crates/argh"
+    }
+  ]
+}