Single thread fake token fetch

src/core/lib/security/credentials/fake/fake_credentials.c

@@ -113,9 +113,12 @@ static void md_only_test_get_request_metadata(
   if (c->is_async) {
     grpc_credentials_metadata_request *cb_arg =
         grpc_credentials_metadata_request_create(creds, cb, user_data);
-    grpc_executor_push(
-        grpc_closure_create(on_simulated_token_fetch_done, cb_arg),
-        GRPC_ERROR_NONE);
+    grpc_timer_init(exec_ctx, &c->timer,
+                    gpr_time_add(gpr_now(GPR_CLOCK_MONOTONIC),
+                                 gpr_time_from_micros(10, GPR_TIMESPAN)),
+                    on_simulated_token_fetch_done, cb_arg,
+                    gpr_now(GPR_CLOCK_MONOTONIC));
+
   } else {
     cb(exec_ctx, user_data, c->md_store->entries, 1, GRPC_CREDENTIALS_OK, NULL);
   }

src/core/lib/security/credentials/fake/fake_credentials.h

@@ -34,6 +34,7 @@
 #ifndef GRPC_CORE_LIB_SECURITY_CREDENTIALS_FAKE_FAKE_CREDENTIALS_H
 #define GRPC_CORE_LIB_SECURITY_CREDENTIALS_FAKE_FAKE_CREDENTIALS_H
 
+#include "src/core/lib/iomgr/timer.h"
 #include "src/core/lib/security/credentials/credentials.h"
 
 /* -- Fake transport security credentials. -- */
@@ -50,6 +51,7 @@ grpc_server_credentials *grpc_fake_transport_security_server_credentials_create(
 typedef struct {
   grpc_call_credentials base;
   grpc_credentials_md_store *md_store;
+  grpc_timer timer;
   int is_async;
 } grpc_md_only_test_credentials;