Fix & test for issue #4288 (unicode surrogate character in Python exception message).

[rwgk]

Description

Closes#4288. See there for background.

While at it:

• Also adding test_error_already_set_message_with_malformed_utf8() (passes without changes).
• Removing a stray block of code (oversight in error_already_set::what() is now constructed lazily #1895).

Suggested changelog entry:

Bug fix: Unicode surrogate character in Python exception message leads to process termination in `error_already_set::what()`.

[henryiii]

PyPy 3.7 Windows gave:

MSBuild version 17.3.1+2badb37d1 for .NET Framework
Checking File Globs
cross_module_gil_utils.vcxproj -> D:\a\pybind11\pybind11\build2\tests\cross_module_gil_utils.pypy37-pp73-win_amd64.pyd
cross_module_interleaved_error_already_set.vcxproj -> D:\a\pybind11\pybind11\build2\tests\cross_module_interleaved_error_already_set.pypy37-pp73-win_amd64.pyd
pybind11_cross_module_tests.vcxproj -> D:\a\pybind11\pybind11\build2\tests\pybind11_cross_module_tests.pypy37-pp73-win_amd64.pyd
pybind11_tests.vcxproj -> D:\a\pybind11\pybind11\build2\tests\pybind11_tests.pypy37-pp73-win_amd64.pyd
------ pybind11_tests.pypy37-pp73-win_amd64.pyd file size: 23571968 (no change)
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(247,5): error MSB8066: Custom build for 'D:\a\pybind11\pybind11\build2\CMakeFiles\c74e89d6f5f1ea108a7628c93f4b5083\pytest.rule;D:\a\pybind11\pybind11\tests\CMakeLists.txt' exited with code -1073741819. [D:\a\pybind11\pybind11\build2\tests\pytest.vcxproj]

That's the expected flake, or something else? Either way, I can rerun. You can access old runs at the top of the checks screen, there's a selectable run number there.

[rwgk]

PyPy 3.7 Windows gave:
That's the expected flake, or something else?

Something else. I've seen it for the first time 2-3 months ago. I don't know what's behind it, at all. But clearly not related to this PR.