Move package into src

Prevents accidental imports when running tests.
diff --git a/tests/test_util.py b/tests/test_util.py
new file mode 100644
index 0000000..2aaded2
--- /dev/null
+++ b/tests/test_util.py
@@ -0,0 +1,19 @@
+from OpenSSL._util import exception_from_error_queue, lib
+
+from .util import TestCase
+
+
+class ErrorTests(TestCase):
+    """
+    Tests for handling of certain OpenSSL error cases.
+    """
+    def test_exception_from_error_queue_nonexistent_reason(self):
+        """
+        :py:func:`exception_from_error_queue` raises ``ValueError`` when it
+        encounters an OpenSSL error code which does not have a reason string.
+        """
+        lib.ERR_put_error(lib.ERR_LIB_EVP, 0, 1112, b"", 10)
+        exc = self.assertRaises(
+            ValueError, exception_from_error_queue, ValueError
+        )
+        self.assertEqual(exc.args[0][0][2], "")