While pathes to executables (PATH) and plugins (QT_PLUGIN_PATHS) can be adjusted, LIBEXEC is hardcoded, see e.g.:

https://community.kde.org/Frameworks/Porting_Notes/KStandardDirs

KStandardDirs::findExe (and the use of the "exe" resource) can be ported to QStandardPaths::findExecutable, except when the executable comes from libexec, in which case the path should just be hardcoded using CMAKE_INSTALL_PREFIX "/" LIBEXEC_INSTALL_DIR.

This makes it impossible to test any private helpers without installing them. Worse, this may give false positives are hide errors when running tests from inside the build directory.