Add a `<Time>` element to Upload.xml that contains the Unix timestamp
when this file was generated. This matches the data recorded by CTest in its
other XML files.
For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
