Enable test preparation targets by default. Add (better named)
`CMAKE_SKIP_TEST_PREP_TARGETS` variable to control generation of the
same, mirroring `CMAKE_SKIP_INSTALL_PREP_TARGETS`. Remove support for
old `CMAKE_TEST_BUILD_DEPENDS` variable. Update and improve
documentation.
This improves consistency between test and install preparation targets.
It also modifies the descriptions of preparation targets in the
documentation of generators which support them to be consistent.
(Previously, each generator described preparation targets in a manner
unique to each generator.)
Fixes: #28050
- Update discover_tests to work with test_prep targets and ctest --out-of-date
- Extend behavior to gtest_discover_tests
- Don't treat non-built targets as always out of date
- Support imported targets in BUILD_DEPENDS
Issue: #28052, #28070
Matthew Woehlke