Update TEST_MAPPING

Test: cd external/rust/crates && atest --host -c
Change-Id: Idd5c5b0e4e7d1c7521cafa1e4c48f6ade5ea986c
diff --git a/TEST_MAPPING b/TEST_MAPPING
index c934591..a0b6beb 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -2,6 +2,9 @@
 {
   "imports": [
     {
+      "path": "external/rust/crates/argh"
+    },
+    {
       "path": "external/rust/crates/base64"
     },
     {