support for timeout

There is no support for timeout in gtest, and the simplest way is to use async task timeout. However, the thread running the async task will block the fuzzer from knowing the task has failed, and not print the failing test case.

My current workaround is to use pthread kill to kill the async task thread, but this is only safe in some cases and platform specific. It would be better if this library can provide a way of handling timeout.