Fix fail_second_ddos_callback flag.

It was failing only on 32-bit for some reason. Part of TestConfig wasn't
getting initialized.

Change-Id: I2a3747a347a47b47e2357f34d32f8db86d6cc629
Reviewed-on: https://boringssl-review.googlesource.com/4110
Reviewed-by: Adam Langley <agl@google.com>