add tests for pkce

Closes #27.

tests/test_pkce.py

@@ -0,0 +1,33 @@
+import pytest
+
+from simple_openid_connect import pkce
+
+
+def test_get_code_challenge():
+    PKCE_PAIRS = [
+        # (verifier, challenge) pairs
+        (
+            "X1mcNAU_lfu25acaJoCQSyo4YyZc1NwaxWw7tNL7mffK0AXCS-lNBtisryb2",
+            "V_nSilRCu0pt3eR-cH7LWru-rYwlTr3J2143tiwloCA",
+        ),
+        (
+            "9ok-JCOrX1OkmCFy8wGq2UBTGgFkeEg5IGffzgKNfDrpO-AOJ-83J9IhXOqj",
+            "ET7UYFBGIHCdYh9bLyeLqWiKEr3t3JCxMRNacq3NCGk",
+        ),
+        (
+            "zDv_Uz7QRoYRgIdieHac1UIlDNPPQb8qVIwgLzFu66kHo2g92UfkoU8Vi91z",
+            "iWLceDSH1e3G7NJ5eieiqpk9IgXPTgSUd8ivdbRZKII",
+        ),
+    ]
+    for verifier, challenge in PKCE_PAIRS:
+        assert (
+            pkce.get_code_challenge(verifier) == challenge
+        ), "get_code_challenge() produced an unexpected challenge"
+
+
+def test_gen_pair_is_actual_pair():
+    for _ in range(100):
+        verifier, challenge = pkce.generate_pkce_pair()
+        assert challenge == pkce.get_code_challenge(
+            verifier
+        ), "pkce.generate_pkce_pair() returned a pair whose challenge cannot be reproduced from the verifier"