Change @isTestGroup to @istest on testGoldens

This method represents a test, not a group of tests. This annotation is used by editors like VS Code to locate tests/groups and merges them with the test results in the tree. Using `isTestGroup` would cause the analysis-based test discovery to add a group with this name, whereas the test results would add a test with this name. Both should be the same.

packages/golden_toolkit/lib/src/testing_tools.dart

@@ -145,7 +145,7 @@ const Object _defaultTagObject = Object();
 ///
 /// [test] test body
 ///
-@isTestGroup
+@isTest
 void testGoldens(
   String description,
   Future<void> Function(WidgetTester) test, {