In gtest_discover_tests(), the TIMEOUT keyword was making it
impossible to set the TIMEOUT test property via the PROPERTIES
keyword. This would be a frequent case, but it doesn't complain
and instead silently does something different to what would
normally be expected. The TIMEOUT keyword has been renamed
to DISCOVERY_TIMEOUT, thereby removing the clash.
This is a breaking change. 3.10.1 and 3.10.2 were the only versions
that supported the TIMEOUT keyword and uses of it were likely
not working as intended.
Fixes: #17801
Add a TIMEOUT option to gtest_discover_tests. This provides a
work-around in case a test goes out to lunch, rather than causing the
build to simply hang. (Although this is still a problem with the user's
project, hanging the build is not cool, especially in the case of
automatically running CI builds. It is much preferred that the build
should actively fail in this case, and it is trivially easy for us to
implement that.)
