Add a stub Rust crate (#12595)

tests/test_rust.py

@@ -0,0 +1,11 @@
+from synapse.synapse_rust import sum_as_string
+
+from tests import unittest
+
+
+class RustTestCase(unittest.TestCase):
+    """Basic tests to ensure that we can call into Rust code."""
+
+    def test_basic(self):
+        result = sum_as_string(1, 2)
+        self.assertEqual("3", result)