Konstantin Varlamov

Rework how tests deal with future timeout

3

Also fix a potential bug in `DescribeFailedFuture` (`error_message()` can return null).

The following variable appears to be unused: https://github.com/docopt/docopt.cpp/blob/400e6dd8e59196c914dcc2c56caf7dae7efa5eb3/docopt_private.h#L549 For context, I work on libc++ and we are considering enabling the unused variable warnings for some standard library types, including `std::string`....