Add unit test for pskproxy

Rudamentary, as it only checks gen_psk() and not ssl.wrap_socket (and as
such won't catch issues like sslpsk#16), but the best we can do for now
and without modifying the code itself.

tests/test_psk.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+"""Test the smarthack.pskproxy module."""
+
+# Copyright (c) 2020 Faidon Liambotis
+# SPDX-License-Identifier: MIT
+
+import pytest  # type: ignore
+
+from smarthack.pskproxy import IDENTITY_PREFIX, gen_psk
+
+
+PREFIX = b"\x01" + IDENTITY_PREFIX
+HINT = b"1dHRsc2NjbHltbGx3eWh5" b"0000000000000000"  # taken from the code being tested
+PRECOMPUTED = (
+    ("", "310ab75a8d067ccea734482eb07adf04"),
+    ("00" * 16, "3100cad5df5a8f719407b55e0ff55e883c53ab665efe02bd1bb93d7eda9e15e2"),
+    ("80" * 16, "36ac9fe6f25bb46b652eeb78c9dbf2c6675f82bf574808da316295cab811efc5"),
+    (
+        "fef0f7f7d1fff1602f64cbdb482d86dc5f35949857a88ba085f9c17b7cae02ed95",
+        "5578f55f9e82e8ab7ea053cbacc7a3f4cc1cfd51ac202a447c5e3155bf347078",
+    ),
+)
+
+
+def test_gen_psk() -> None:
+    """Test the gen_psk() method against precomputed data."""
+    for clear, encrypted in PRECOMPUTED:
+        identity = PREFIX + bytes.fromhex(clear)
+        assert gen_psk(identity, HINT).hex() == encrypted
+
+    with pytest.raises(ValueError):
+        assert gen_psk(b"not 16 chars", HINT)