Improve tests (#1918)

<!-- Write down your pull request descriptions. -->

- Change `SocketException on request` to be platform-independent.
- Improve the readability of `SocketException on response`.

### New Pull Request Checklist

- [x] I have read the
[Documentation](https://pub.dev/documentation/dio/latest/)
- [x] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
- [x] I have updated this branch with the latest `main` branch to avoid
conflicts (via merge from master or rebase)
- [x] I have added the required tests to prove the fix/feature I'm
adding
- [ ] I have updated the documentation (if necessary)
- [x] I have run the tests without failures
- [ ] I have updated the `CHANGELOG.md` in the corresponding package

### Additional context and info (if any)

<!-- Provide more context and info about the PR. -->

Co-authored-by: Flop <[email protected]>

dio/test/stacktrace_test.dart

@@ -238,7 +238,15 @@ void main() async {
         () async {
           final dio = Dio()
             ..options.baseUrl = 'https://does.not.exist'
-            ..httpClientAdapter = IOHttpClientAdapter();
+            ..httpClientAdapter = IOHttpClientAdapter(
+              createHttpClient: () {
+                final client = MockHttpClient();
+                when(client.openUrl(any, any)).thenThrow(
+                  SocketException('test'),
+                );
+                return client;
+              },
+            );
 
           await expectLater(
             dio.get('/test', data: 'test'),
@@ -247,9 +255,6 @@ void main() async {
                 isA<DioException>(),
                 (e) => e.type == DioExceptionType.unknown,
                 (e) => e.error is SocketException,
-                (e) => (e.error as SocketException)
-                    .message
-                    .startsWith("Failed host lookup: 'does.not.exist'"),
                 (e) => e.stackTrace
                     .toString()
                     .contains('test/stacktrace_test.dart'),
@@ -273,8 +278,9 @@ void main() async {
                 when(request.headers).thenReturn(MockHttpHeaders());
                 when(request.addStream(any)).thenAnswer((_) => Future.value());
                 when(request.close()).thenAnswer(
-                  (_) async => Future.delayed(Duration(milliseconds: 50),
-                      () => throw SocketException('test')),
+                  (_) => Future.delayed(Duration(milliseconds: 50), () {
+                    throw SocketException('test');
+                  }),
                 );
                 return client;
               },