Disable 20s unittest timeout for docs_test and dartdoc_test.

sdk/lib/_internal/dartdoc/test/dartdoc_test.dart

Index: sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
diff --git a/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart b/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
index 4aa4935213af70d1fffdef33bfc64f17a954b5cb..be14aa095e94110befffb75cace2cbada11c822c 100644
--- a/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
+++ b/sdk/lib/_internal/dartdoc/test/dartdoc_test.dart
@@ -17,6 +17,8 @@ import '../lib/markdown.dart';
 import 'markdown_test.dart';
 
 main() {
+  // Some tests take more than the default 20 second unittest timeout.
+  unittestConfiguration.timeout = null;
   group('isAbsolute', () {
     final doc = new dd.Dartdoc();