test: Use correct option for test discovery timeout

From: https://cmake.org/cmake/help/latest/module/GoogleTest.html#DISCOVERY_TIMEOUT

> In CMake versions 3.10.1 and 3.10.2, this option was called TIMEOUT.
> This clashed with the TIMEOUT test property, which is one of the common
> properties that would be set with the PROPERTIES keyword, usually
> leading to legal but unintended behavior. The keyword was changed to
> DISCOVERY_TIMEOUT in CMake 3.10.3 to address this problem. The ambiguous
> behavior of the TIMEOUT keyword in 3.10.1 and 3.10.2 has not been
> preserved.