bpo-29845: Mark tests that use _testcapi as CPython-only (#711) (#725)

(cherry picked from commit 24c738a9e9)

Lib/test/test_tracemalloc.py

@@ -865,6 +865,7 @@ class TestCommandLine(unittest.TestCase):
                                   b'number of frames',
                                   stderr)
 
+    @unittest.skipIf(_testcapi is None, 'need _testcapi')
     def test_pymem_alloc0(self):
         # Issue #21639: Check that PyMem_Malloc(0) with tracemalloc enabled
         # does not crash.