Start using /list_tests to populate subtest menus in test-picker

dashboard/dashboard/elements/test-picker-test.html

Index: dashboard/dashboard/elements/test-picker-test.html
diff --git a/dashboard/dashboard/elements/test-picker-test.html b/dashboard/dashboard/elements/test-picker-test.html
index e663c116f6842d32a3eae6eac77763e3a5a1613e..b750eee679c43ecb4811685d19ecb601cc335a62 100644
--- a/dashboard/dashboard/elements/test-picker-test.html
+++ b/dashboard/dashboard/elements/test-picker-test.html
@@ -66,7 +66,10 @@ tr.b.unittest.testSuite(function() {
 
   test('instantiate', function() {
     testing_common.addXhrMock('*', JSON.stringify(MOCK_RESPONSE));
-    var testPicker = document.createElement('test-picker');
+    const testPicker = document.createElement('test-picker');
+    testPicker.subtests.prepopulate({
+
+    });
     testPicker.testSuites = {
       'endure': {
         'mas': {'Chromium': {'mac': false, 'win7': true}},