Add `test_prep/<test_name>` convenience targets to the ninja generator
to build dependencies of tests, and `test_prep/all` to build the dependencies
of all tests.
Build dependencies of a test include:
- Executables directly invoked by test `COMMAND`.
- Targets mentioned in generator expressionf os test `COMMAND`.
- Anything explicitly listed with the new `BUILD_DEPENDS` argument of `add_test`.
Issue: #27613