Add test_namespace_visibility (experiment, not for merging)

[rwgk]

Description

To answer the question: Is there a difference in behavior if some __attribute__((visibility("hidden"))) are missing (PR #4043)?

A second motivation for this experiment was to gain clarity about Google-internal vs external behavior.

Suggested changelog entry:

[rwgk]

Summary of results for commit 3f01275:

• https://github.com/pybind/pybind11/actions/runs/2622665174

• logs_21183.zip

$ grep UNEXPECTED *.txt | cut -d']' -f2- | cut -d' ' -f3- | sort | uniq -c
      6 UNEXPECTED code_line: AAA:AAc:AaA:Aac:aAC:aAc:aaC:aac
     30 UNEXPECTED code_line: AAA:AAc:AaC:Aac:aAC:aAc:aaC:aac

$ grep 'UNEXPECTED code_line: AAA:AAc:AaA:Aac:aAC:aAc:aaC:aac' *.txt | cutniq
13______3.6_____macos-latest_____x64.txt
14______3.9_____macos-latest_____x64.txt
15______3.10_____macos-latest_____x64.txt

$ grep 'UNEXPECTED code_line: AAA:AAc:AaC:Aac:aAC:aAc:aaC:aac' *.txt | cutniq
1______3.6_____ubuntu-latest_____x64_-DPYBIND11_FINDPYTHON=ON_-DCM.txt
1______3.7_____Debian_____x86______Install.txt
1______3.9-dbg__deadsnakes______Valgrind_____x64.txt
1______3_____centos7_____x64.txt
1______3_____Clang_3.6_____C++11_____x64.txt
1______3_____GCC_7_____C++11____x64.txt
2______3.9_____ubuntu-latest_____x64.txt
2______3_____almalinux8_____x64.txt
2______3_____Clang_3.7_____C++11_____x64.txt
2______3_____GCC_latest_____C++11____x64.txt
3______3.10_____ubuntu-latest_____x64.txt
3______3_____almalinux9_____x64.txt
3______3_____Clang_3.9_____C++11_____x64.txt
3______3_____GCC_10_____C++20____x64.txt
4______3_____Clang_7_____C++11_____x64.txt
4______pypy-3.7_____ubuntu-latest_____x64.txt
5______3_____Clang_9_____C++11_____x64.txt
5______pypy-3.8_____ubuntu-latest_____x64_-DPYBIND11_FINDPYTHON=ON.txt
6______3_____Clang_dev_____C++11_____x64.txt
6______pypy-3.9_____ubuntu-latest_____x64.txt
7______3_____Clang_5_____C++14_____x64.txt
8______3_____Clang_10_____C++20_____x64.txt
9______3_____Clang_10_____C++17_____x64.txt

[rwgk]

To explain:

      6 UNEXPECTED code_line: AAA:AAc:AaA:Aac:aAC:aAc:aaC:aac
     30 UNEXPECTED code_line: AAA:AAc:AaC:Aac:aAC:aAc:aaC:aac
----------------------------------------^

The dynamic loaders behave differently on macOS (1st line above) vs Linux (2nd line).

If a namespace has hidden visibility in one Translation Unit (TU), but default in another TU linked into the same PYBIND11_MODULE, the net result is that it is

• macOS: visible from another PYBIND11_MODULE.
• Linux: hidden from another PYBIND11_MODULE.

Note that this is withOUT -fvisibility=hidden and RTLD_GLOBAL on Linux, i.e. for an unusual situation.

[rwgk]

The code added here was moved to separate files (tests/namespace_visibility) under PR #4050.