test_busy_poll.cpp missing from zeromq 4.3.5 tarball

[tajmorton]

Issue description

The zeromq 4.3.5 release tarballs linked from the releases page on Github are missing the tests/test_busy_poll.cpp file.
https://github.com/zeromq/libzmq/releases/tag/v4.3.5

This causes build failures when building on a platform with SO_BUSY_POLL/ZMQ_HAVE_BUSY_POLL:
CMake Error:

Cannot find source file:
test_busy_poll.cpp
[...]
CMake Error at tests/CMakeLists.txt:267 (add_executable):
  No SOURCES given to target: test_busy_poll

Environment

• libzmq version (commit hash if unreleased): 4.3.5
• OS: Linux (Ubuntu 22.04 Docker Image)

Minimal test code / Steps to reproduce the issue

1. wget https://github.com/zeromq/libzmq/releases/download/v4.3.5/zeromq-4.3.5.tar.gz
2. tar xvf zeromq-4.3.5.tar.gz
3. cd zeromq-4.3.5/
4. grep -C1 test_busy_poll.cpp tests/CMakeLists.txt

  if(ZMQ_HAVE_BUSY_POLL)
    list(APPEND tests test_busy_poll)
  endif()

5. ls tests/test_busy_poll.cpp

ls: tests/test_busy_poll.cpp: No such file or directory

What's the actual result?

test_busy_poll.cpp is not present in release tarball, so unit test cannot be added to CMake build.

The file is present on the v4.3.5 Git tag: https://github.com/zeromq/libzmq/blob/v4.3.5/tests/test_busy_poll.cpp

What's the expected result?

tests/test_busy_poll.cpp exists in release tarball.