gh-144490: Fix test_cppext: test the internal C API

[vstinner]

Add missing TEST_INTERNAL_C_API env var.

• Issue: Enhancement request: make pycore_cell.h compile cleanly under C++ #144490

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit c1efd83 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F144536%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit 37db34b 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F144536%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[hugovk]

test.test_cppext.TestInteralCAPI.test_build has started failing on the "AMD64 Windows11 Non-Debug 3.x" buildbot after merging this:

• https://buildbot.python.org/#/release_status
• https://buildbot.python.org/#/builders/914/builds/5444

I see this PR did pass the buildbots, including "AMD64 Windows11 Non-Debug PR":

https://buildbot.python.org/#/builders/1213/builds/887

Oh but wait, that build didn't actually complete:

fatal: couldn't find remote ref refs/pull/144536/merge
program finished with exit code 128

That's not very helpful to be reported as a pass...

Anyway, please could you check the test failure? Reminder we have the 3.15 alpha on Tuesday.

Thanks!

[vstinner]

test.test_cppext.TestInteralCAPI.test_build has started failing on the "AMD64 Windows11 Non-Debug 3.x" buildbot after merging this

Oh sorry. It seems like I messed up with my local tests.

I wrote #144628 to fix test_cppext.