tests: fix IsolatedConfig test

[b-pass]

The test was throwing import errors past the lifetime of the owning scoped_interpreter

Description

The test was supposed to import a test module, but because it is "isolated", the path is not set, so the module cannot be imported. A ModuleNotFound error is thrown, but not caught, and then propagates past the scoped_interpreter which owns it.

The stack of the SEGV doesn't point to the test itself, so it was difficult to track down:

#0  0x0000000004b6fec9 in new_threadstate (interp=0x0) at ../Python/pystate.c:1388
#1  0x0000000004b726ad in PyGILState_Ensure () at ../Python/pystate.c:2218
#2  0x0000000000162821 in pybind11::gil_scoped_acquire_simple::gil_scoped_acquire_simple (this=0x1ffeffeb8c)
    at /home/user/pybind11/include/pybind11/gil_simple.h:17
#3  0x0000000000163cc8 in pybind11::error_already_set::what (this=0x6f58d60) at /home/user/pybind11/include/pybind11/pybind11.h:3345
#4  0x000000000012dd32 in Catch::ExceptionTranslatorRegistry::translateActiveException[abi:cxx11]() const (this=0x5729138)
    at /home/user/pybind11/build12/tests/catch/catch.hpp:10720
#5  0x00000000001343ca in Catch::translateActiveException[abi:cxx11]() () at /home/user/pybind11/build12/tests/catch/catch.hpp:12523
#6  0x00000000001369e0 in Catch::RunContext::runCurrentTest (this=0x1ffefff1f0, redirectedCout="", redirectedCerr="")
    at /home/user/pybind11/build12/tests/catch/catch.hpp:13008
#7  0x0000000000135236 in Catch::RunContext::runTest (this=0x1ffefff1f0, testCase=...) at /home/user/pybind11/build12/tests/catch/catch.hpp:12759
#8  0x0000000000138533 in Catch::(anonymous namespace)::TestGroup::execute (this=0x1ffefff1e0) at /home/user/pybind11/build12/tests/catch/catch.hpp:13352
#9  0x0000000000139abb in Catch::Session::runInternal (this=0x1ffefff510) at /home/user/pybind11/build12/tests/catch/catch.hpp:13562
#10 0x000000000013978e in Catch::Session::run (this=0x1ffefff510) at /home/user/pybind11/build12/tests/catch/catch.hpp:13518
#11 0x00000000001841af in Catch::Session::run<char> (this=0x1ffefff510, argc=2, argv=0x1ffefff7e8) at /home/user/pybind11/build12/tests/catch/catch.hpp:13236
#12 0x00000000001521bb in main (argc=2, argv=0x1ffefff7e8) at /home/user/pybind11/tests/test_embed/catch.cpp:40

I do not really understand how this passes as it is.

@rwgk this is what I found when investigating the flake from #5766. I don't know if this is the actual cause....

Suggested changelog entry:

• fix an IsolatedConfig test

[rwgk]

@b-pass There was this flake again (resolved by a rerun):

https://github.com/pybind/pybind11/actions/runs/16509608392/job/46688698596?pr=5768

D:/a/_temp/msys64/mingw64/bin/mingw32-make.exe  -f tests\test_embed\CMakeFiles\cpptest.dir\build.make tests/test_embed/CMakeFiles/cpptest.dir/depend
mingw32-make[3]: Entering directory 'D:/a/pybind11/pybind11/build2'
D:\a\_temp\msys64\mingw64\bin\cmake.exe -E cmake_depends "MinGW Makefiles" D:\a\pybind11\pybind11 D:\a\pybind11\pybind11\tests\test_embed D:\a\pybind11\pybind11\build2 D:\a\pybind11\pybind11\build2\tests\test_embed D:\a\pybind11\pybind11\build2\tests\test_embed\CMakeFiles\cpptest.dir\DependInfo.cmake "--color="
mingw32-make[3]: Leaving directory 'D:/a/pybind11/pybind11/build2'
D:/a/_temp/msys64/mingw64/bin/mingw32-make.exe  -f tests\test_embed\CMakeFiles\cpptest.dir\build.make tests/test_embed/CMakeFiles/cpptest.dir/build
mingw32-make[3]: Entering directory 'D:/a/pybind11/pybind11/build2'
cd /d D:\a\pybind11\pybind11\build2\tests\test_embed && D:\a\pybind11\pybind11\build2\tests\test_embed\test_embed.exe
Exception ignored in: <module 'threading' from 'D:/a/_temp/msys64/mingw64/lib/python3.12/threading.py'>
Traceback (most recent call last):
  File "D:/a/_temp/msys64/mingw64/lib/python3.12/threading.py", line 1602, in _shutdown
    assert tlock.locked()
           ^^^^^^^^^^^^^^
AssertionError: 
Windows fatal exception: access violation

For context, from a little further up in the log:

============================= test session starts =============================
platform win32 -- Python 3.12.11, pytest-8.4.0, pluggy-1.6.0
installed packages of interest: numpy==2.3.1 scipy==1.15.3
C++ Info: 15.1.0 C++14 __pybind11_internals_v11_mingw_libstdcpp_gxx_abi_1xxx_use_cxx11_abi_1__ PYBIND11_SIMPLE_GIL_MANAGEMENT=False

I.e. similar to what I saw (and reported elsewhere) before, this is with Python 3.12.

[rwgk]

Thanks for fixing this!