CMake execute_process() doesn't print till program ends or stdout is flushed

CMakeLists.txt

cmake_minimum_required(VERSION 3.12)

project(stdout_buffering LANGUAGES CXX)
enable_testing()

set(CMAKE_CXX_STANDARD 11)

add_executable(print_flush print_flush.cpp)
add_test(NAME print_flush COMMAND print_flush)

find_package(Python COMPONENTS Interpreter)
if(Python_FOUND)
    add_test(NAME print_flush_python
    COMMAND ${Python_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/print_flush.py)
endif()

set_property(TEST print_flush print_flush_python PROPERTY TIMEOUT 10)

file(GENERATE OUTPUT .gitignore CONTENT "*")

execute_process_stdout.cmake

# Observe for Python flush=False, the printing is buffered till the first flush=True

cmake_minimum_required(VERSION 3.19)

find_package(Python COMPONENTS Interpreter REQUIRED)

execute_process(COMMAND ${Python_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/print_flush.py
RESULT_VARIABLE ret
)
if(ret EQUAL 0)
  message(STATUS "OK: print_flush")
else()
  message(FATAL_ERROR "print_flush failed:  ${ret}")
endif()

print_flush.cpp

// https://en.cppreference.com/w/cpp/io/manip/endl
// https://en.cppreference.com/w/cpp/io/cout
// https://en.cppreference.com/w/cpp/io/ios_base/sync_with_stdio

#include <chrono>
#include <thread>
#include <iostream>
#include <cstdlib>


int main(){

int milliseconds = 200, N = 6;

std::cout << "flush: True" << std::endl;
for (int i = 0; i < N; ++i) {
  std::this_thread::sleep_for(std::chrono::milliseconds(milliseconds));
  std::cout << (float(i) / N) * 100 << " %" << std::endl;
}

std::cout << "flush: False\n";
std::cout.sync_with_stdio(false);
// on some platforms, stdout flushes on \n
for (int i = 0; i < N; ++i) {
  std::this_thread::sleep_for(std::chrono::milliseconds(milliseconds));
  std::cout << (float(i) / N) * 100 << " %\n";
}

return EXIT_SUCCESS;

}

print_flush.py

#!/usr/bin/env python3
"""
print() without flush and with flush

https://docs.python.org/3/library/functions.html#print
"""

from time import sleep

N = 6  # arbitrary number of prints

for flush in (True, False):
    print("flush:", flush)
    for i in range(N):
        sleep(0.2)
        print(f"{i/N*100:.1f} %", flush=flush)