bpo-44133: Skip PyThread_get_thread_native_id() if not available (GH-30636)

test_capi.test_export_symbols() doesn't check if Python exports the
"PyThread_get_thread_native_id" symbol if the _thread.get_native_id()
function is not available (if the PY_HAVE_THREAD_NATIVE_ID macro is
not defined).