Add fmtlib to hwasan-postsubmit.

Change-Id: I1610f11b79f1d5e612c95d9ac7467e3b5f6303ac
Test: Run tests under HWASan.
Bug: 193568145
diff --git a/TEST_MAPPING b/TEST_MAPPING
index caeecd6..328028d 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -6,5 +6,13 @@
     {
       "name": "fmtlib_ostream_test"
     }
+  ],
+  "hwasan-postsubmit": [
+    {
+      "name": "fmtlib_test"
+    },
+    {
+      "name": "fmtlib_ostream_test"
+    }
   ]
 }