Run test tearDowns and clean up temporary directories when a signal is caught.

test/runner/browser/runner_test.dart

Index: test/runner/browser/runner_test.dart
diff --git a/test/runner/browser/runner_test.dart b/test/runner/browser/runner_test.dart
index abe5ac4da5e06f56e21b6a64fdb5e8e26f0a81f6..aea05886d3756e828653b590adb81018a3dcd6fa 100644
--- a/test/runner/browser/runner_test.dart
+++ b/test/runner/browser/runner_test.dart
@@ -8,6 +8,7 @@ import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:test/src/util/exit_codes.dart' as exit_codes;
+import 'package:test/src/util/io.dart';
 import 'package:test/test.dart';
 
 import '../../io.dart';
@@ -26,7 +27,7 @@ void main() {
 
 void main() {
   setUp(() {
-    _sandbox = Directory.systemTemp.createTempSync('test_').path;
+    _sandbox = createTempDir();
   });
 
   tearDown(() {