test: add sign message test PE-1136

Author: javdhu

test/wallets_test.dart

@@ -1,7 +1,9 @@
 import 'dart:convert';
 import 'dart:io';
+import 'dart:typed_data';
 
 import 'package:arweave/arweave.dart';
+import 'package:arweave/utils.dart';
 import 'package:test/test.dart';
 
 import 'utils.dart';
@@ -55,5 +57,18 @@ void main() {
     }, onPlatform: {
       'browser': Skip('dart:io unavailable'),
     });
+
+    test('sign message with wallet', () async {
+      final wallet = await getTestWallet();
+      final message = utf8.encode('<test message>');
+
+      final signature = await wallet.signMessage(message as Uint8List);
+      expect(
+        encodeBytesToBase64(signature),
+        startsWith('II5LxGnPt4WTSz9P__wMAdjzXWlZE-wGbKU7wm4DbGuPXB5Vifs'),
+      );
+    }, onPlatform: {
+      'browser': Skip('dart:io unavailable'),
+    });
   });
 }