remove mnemonic from sharedSetup

Author: AlaaElattar

packages/tfchain_client/test/nodes_test.dart

@@ -1,3 +1,5 @@
+import 'dart:io';
+
 import 'package:test/test.dart';
 import 'package:tfchain_client/generated/dev/types/tfchain_support/types/node.dart';
 import 'package:tfchain_client/tfchain_client.dart';
@@ -68,7 +70,6 @@ void main() {
       }
     });
 
-    // TODO:
     tearDownAll(() async {
       await client.disconnect();
     });

packages/tfchain_client/test/shared_setup.dart

@@ -48,8 +48,7 @@ String generateRandomString(int length) {
 
 void sharedSetup() {
   setUp(() {
-    mnemonic = Platform.environment['MNEMONIC'] ??
-        "secret add bag cluster deposit beach illness letter crouch position rain arctic";
+    mnemonic = Platform.environment['MNEMONIC']!;
     url = Platform.environment['URL'] ?? 'wss://tfchain.dev.grid.tf/ws';
     type = Platform.environment['KEYPAIR_TYPE'] ?? 'sr25519';