Add a `COMMAND_EXPAND_LISTS` option to the `add_test` command to cause
`;`-separated lists produced by generator expressions to be expanded
into multiple arguments. The `add_custom_command` command already
has such an option.
Fixes: #17284
Generator expressions are supported in many places and are a distinct
concept worthy of their own manual page. The old builtin documentation
was previously represented by preprocessor macros to generate it into
each place that supports them. Factor out the duplicate content into a
dedicated cmake-generator-expressions manual page and reference it from
each original location.