Add test for base64 decoding.

Author: rkistner

packages/powersync/test/credentials_test.dart

@@ -0,0 +1,14 @@
+import 'package:powersync/powersync.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('PowerSyncCredentials', () {
+    test('getExpiryDate', () async {
+      // Specifically test a token with a "-" character and missing padding
+      final token =
+          'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJ9Pn0-YWIiLCJpYXQiOjE3MDc3Mzk0MDAsImV4cCI6MTcwNzczOTUwMH0=.IVoAtpJ7jfwLbqlyJGYHPCvljLis_fHj2Qvdqlj8AQU';
+      expect(PowerSyncCredentials.getExpiryDate(token)?.toUtc(),
+          equals(DateTime.parse('2024-02-12T12:05:00Z')));
+    });
+  });
+}