Add tests to cover these cases:
* run as regular CMake module, in NORMAL_MODE (expected to fail);
* run as CMake script in SCRIPT_MODE (expected to exit with given code);
* run as CMake script that `include()`-s another script with EXIT subcommand;
* run as CMake script which EVAL-uates EXIT subcommand via
`cmake_language(EVAL CODE "<cmake code>")`.
Fixes: #23162
1bb1769235