GoogleTest: Rename TIMEOUT parameter to avoid clash

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 use 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. 3.10.1 gained the TIMEOUT keyword in !1534 (merged).

Fixes: #17801 (closed)