FAILED tests/test_module_attributes.py::TestModuleAttributes::test_features fails with --system-zstd

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```
python3 setup.py test --system-zstd
...
self =

def test_features(self):
self.assertIsInstance(zstd.backend_features, set)

expected = {
"cext": {
"buffer_types",
"multi_compress_to_buffer",
"multi_decompress_to_buffer",
},
"cffi": set(),
"rust": {
"buffer_types",
"multi_compress_to_buffer",
"multi_decompress_to_buffer",
},
}[zstd.backend]

> self.assertEqual(zstd.backend_features, expected)
E AssertionError: Items in the second set but not the first:
E 'multi_compress_to_buffer'
E 'multi_decompress_to_buffer'

tests/test_module_attributes.py:29: AssertionError
```

I guess the tested features are available only with a statically linked copy of libzstd, right? The tests should reflect that.

Start with tests/test_module_attributes.py::TestModuleAttributes::test_features and reproduce the failure using `python3 setup.py test --system-zstd`. Compare the reported backend features with the system-zstd configuration, then update the test expectations so they match supported features and confirm the test suite passes.

Written by the indexing model from the issue text.