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

test/runner/loader_test.dart

Index: test/runner/loader_test.dart
diff --git a/test/runner/loader_test.dart b/test/runner/loader_test.dart
index 9be28d77ff7ebf23b264a90dd39dece8e810ba73..f56bc94b4a9a1871c743f4c6814ed7780baac477 100644
--- a/test/runner/loader_test.dart
+++ b/test/runner/loader_test.dart
@@ -10,6 +10,7 @@ import 'package:path/path.dart' as p;
 import 'package:test/src/backend/state.dart';
 import 'package:test/src/backend/test_platform.dart';
 import 'package:test/src/runner/loader.dart';
+import 'package:test/src/util/io.dart';
 import 'package:test/test.dart';
 
 import '../io.dart';
@@ -34,7 +35,7 @@ void main() {
   setUp(() {
     _loader = new Loader([TestPlatform.vm],
         packageRoot: p.join(packageDir, 'packages'));
-    _sandbox = Directory.systemTemp.createTempSync('test_').path;
+    _sandbox = createTempDir();
   });
 
   tearDown(() {