[3.13] gh-151284: Fix test_capi on UBSan (GH-151286) (#151325)

gh-151284: Fix test_capi on UBSan (GH-151286)

Comment two checks relying on undefined behavior in
test_fromwidechar() of test_capi.

Enable test_capi in GitHub Action "Reusable Sanitizer".
(cherry picked from commit d87d772)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: miss-islington

Lib/test/test_capi/test_unicode.py

@@ -796,16 +796,18 @@ def test_fromwidechar(self):
         if SIZEOF_WCHAR_T == 2:
             self.assertEqual(fromwidechar('a\U0001f600'.encode(encoding), 2), 'a\ud83d')
 
-        self.assertRaises(MemoryError, fromwidechar, b'', PY_SSIZE_T_MAX)
         self.assertRaises(SystemError, fromwidechar, b'\0'*SIZEOF_WCHAR_T, -2)
-        self.assertRaises(SystemError, fromwidechar, b'\0'*SIZEOF_WCHAR_T, PY_SSIZE_T_MIN)
         self.assertEqual(fromwidechar(NULL, 0), '')
         self.assertRaises(SystemError, fromwidechar, NULL, 1)
         self.assertRaises(SystemError, fromwidechar, NULL, PY_SSIZE_T_MAX)
         self.assertRaises(SystemError, fromwidechar, NULL, -1)
         self.assertRaises(SystemError, fromwidechar, NULL, -2)
         self.assertRaises(SystemError, fromwidechar, NULL, PY_SSIZE_T_MIN)
 
+        # The following tests are skipped since they rely on undefined behavior
+        #self.assertRaises(MemoryError, fromwidechar, b'', PY_SSIZE_T_MAX)
+        #self.assertRaises(SystemError, fromwidechar, b'\0'*SIZEOF_WCHAR_T, PY_SSIZE_T_MIN)
+
     @support.cpython_only
     @unittest.skipIf(_testlimitedcapi is None, 'need _testlimitedcapi module')
     def test_aswidechar(self):