build _testinternalcapi as a shared module

Author: jjhelmus

cpython-unix/extension-modules.yml

@@ -638,6 +638,7 @@ _testinternalcapi:
     - Include/internal
   sources:
     - _testinternalcapi.c
+  build-mode: shared
   includes-conditional:
     - path: _testinternalcapi/parts.h
       minimum-python-version: "3.13"

src/validation.rs

@@ -840,7 +840,13 @@ const GLOBAL_EXTENSIONS_WINDOWS_PRE_3_13: &[&str] = &["_msi"];
 const GLOBAL_EXTENSIONS_WINDOWS_NO_STATIC: &[&str] = &["_testinternalcapi", "_tkinter"];
 
 /// Extension modules that should be built as shared libraries.
-const SHARED_LIBRARY_EXTENSIONS: &[&str] = &["_crypt", "_ctypes_test", "_dbm", "_tkinter"];
+const SHARED_LIBRARY_EXTENSIONS: &[&str] = &[
+    "_crypt",
+    "_ctypes_test",
+    "_dbm",
+    "_testinternalcapi",
+    "_tkinter",
+];
 
 const PYTHON_VERIFICATIONS: &str = include_str!("verify_distribution.py");