Fix the test failure with callback-based polling.

test/core/transport/chttp2/settings_timeout_test.cc

@@ -196,6 +196,8 @@ class Client {
           "grpc_pollset_work",
           grpc_pollset_work(pollset_, &worker,
                             grpc_core::ExecCtx::Get()->Now() + 1000));
+      // Flushes any work scheduled before or during polling.
+      grpc_core::ExecCtx::Get()->Flush();
       gpr_mu_unlock(mu_);
       if (state != nullptr && state->done()) return true;
       if (grpc_core::ExecCtx::Get()->Now() >= deadline) return false;