gh-95991: Add some infrastructure for testing Limited API in _testcapi (GH-95992)

- Limited API needs to be enabled per source file
- Some builds don't support Limited API, so Limited API tests must be skipped on those builds
  (currently this is `Py_TRACE_REFS`, but that may change.)
- `Py_LIMITED_API` must be defined before `<Python.h>` is included.

This puts the hoop-jumping in `testcapi/parts.h`, so individual
test files can be relatively simple. (Currently that's only
`vectorcall_limited.c`, imagine more.)

2 files changed, 12 insertions, 4 deletions

diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index c51a1f2..2409fb0 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -46,6 +46,7 @@ __all__ = [
     "anticipate_failure", "load_package_tests", "detect_api_mismatch",
     "check__all__", "skip_if_buggy_ucrt_strfptime",
     "check_disallow_instantiation", "check_sanitizer", "skip_if_sanitizer",
+    "requires_limited_api",
     # sys
     "is_jython", "is_android", "is_emscripten", "is_wasi",
     "check_impl_detail", "unix_shell", "setswitchinterval",
@@ -1069,6 +1070,15 @@ def refcount_test(test):
     return no_tracing(cpython_only(test))
 
 
+def requires_limited_api(test):
+    try:
+        import _testcapi
+    except ImportError:
+        return unittest.skip('needs _testcapi module')(test)
+    return unittest.skipUnless(
+        _testcapi.LIMITED_API_AVAILABLE, 'needs Limited API support')(test)
+
+
 def _filter_suite(suite, pred):
     """Recursively filter test cases in a suite based on a predicate."""
     newtests = []
diff --git a/Lib/test/test_call.py b/Lib/test/test_call.py
index 131b45e..c00de27 100644
--- a/Lib/test/test_call.py
+++ b/Lib/test/test_call.py
@@ -1,5 +1,5 @@
 import unittest
-from test.support import cpython_only
+from test.support import cpython_only, requires_limited_api
 try:
     import _testcapi
 except ImportError:
@@ -760,9 +760,7 @@ class TestPEP590(unittest.TestCase):
                 self.assertEqual(expected, meth(*args1, **kwargs))
                 self.assertEqual(expected, wrapped(*args, **kwargs))
 
-    @unittest.skipIf(
-        hasattr(sys, 'getobjects'),
-        "Limited API is not compatible with Py_TRACE_REFS")
+    @requires_limited_api
     def test_vectorcall_limited(self):
         from _testcapi import pyobject_vectorcall
         obj = _testcapi.LimitedVectorCallClass()