unix: enable _testinternalcapi

This effectively reverts commit
5611869.

The lack of this extension was undermining PGO.

Enabling this on 3.11 was non-trivial because its bootstrap Python
didn't want to link with this extension, which is typically built
shared. It was easier to always build a host Python than to coerce
CPython's build system to have its bootstrap Python work with this
extension.

AFAICT Python < 3.11 doesn't have any bootstrap issues with enabling
this module.

Author: indygreg

cpython-unix/extension-modules.yml

@@ -473,15 +473,12 @@ _testimportmultiple:
   sources:
     - _testimportmultiple.c
 
-# Similar deal as _testcapi. Extension exists to test the exported C API. Since
-# we're statically linking, assumptions are invalidated. So value of extension
-# is minimal.
-#
-# If we ever enable this, we should add $(srcdir)/Include/internal to includes,
-# otherwise it likely picks up public API.
+# Ideally we disable all test extensions. However, this one is used by a bunch
+# of tests, including tests that run during PGO profiling. We keep it enabled
+# so it doesn't break tests and undermine PGO.
 _testinternalcapi:
-  disabled-targets:
-    - .*
+  includes:
+    - Include/internal
   sources:
     - _testinternalcapi.c
 

src/validation.rs

@@ -673,6 +673,7 @@ const GLOBAL_EXTENSIONS_POSIX: &[&str] = &[
     "_dbm",
     "_posixshmem",
     "_posixsubprocess",
+    "_testinternalcapi",
     "fcntl",
     "grp",
     "posix",