[tests] Print sub-process output in standalone/io/process_working_directory_test

This test runs sub-process and sometimes fails because sub-process has
an unexpected exit code. This is an attempt to get more information
about the cause of such failures.

Issue: https://github.com/dart-lang/sdk/issues/61713

Change-Id: I38eff88b0f94058510078e6e4862d2ad56df2dd7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/454320
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

tests/standalone/io/process_working_directory_test.dart

@@ -4,9 +4,9 @@
 //
 // Process working directory test.
 
-library ProcessWorkingDirectoryTest;
-
+import 'dart:convert';
 import "dart:io";
+
 import "package:expect/expect.dart";
 import "process_test_util.dart";
 
@@ -35,8 +35,8 @@ class ProcessWorkingDirectoryTest {
             Expect.equals(99, exitCode);
             directory.deleteSync();
           });
-          process.stdout.listen((_) {});
-          process.stderr.listen((_) {});
+          process.stdout.transform(utf8.decoder).listen(stdout.write);
+          process.stderr.transform(utf8.decoder).listen(stderr.write);
         })
         .catchError((error) {
           directory.deleteSync();