Add a support for layout tests.

client/testing/unittest/unittestsuite.dart

Index: client/testing/unittest/unittestsuite.dart
diff --git a/client/testing/unittest/unittestsuite.dart b/client/testing/unittest/unittestsuite.dart
index a12972e44561780ce0eae718b70c788fc383adb3..a1057c818fcb786a3df6b264db53cd4d3d512d57 100644
--- a/client/testing/unittest/unittestsuite.dart
+++ b/client/testing/unittest/unittestsuite.dart
@@ -331,6 +331,11 @@ void callbackDone() {
   _currentSuite.callbackDone();
 }
 
+void forLayoutTests() {
+  _ensureActiveSuite();
+  _currentSuite._isLayoutTest = true;
+}
+
 String _fullSpec(String spec) {
   if (spec === null) return '$_currentGroup';
   return _currentGroup != '' ? '$_currentGroup $spec' : spec;