style: Simplify and unify syntax used for exports on flame_test (#3479)

Simplify and unify syntax used for exports on flame_test.

The file in question only has one public class, which is exported.
This brings it in line with all other exports on the file - the only
exception being `test_golden.dart` which does indeed declare other
auxiliary public types.

packages/flame_test/lib/flame_test.dart

@@ -1,7 +1,7 @@
-export 'src/close_to_aabb.dart' show closeToAabb;
+export 'src/close_to_aabb.dart';
 export 'src/close_to_vector.dart';
 export 'src/close_to_vector3.dart';
-export 'src/debug_text_renderer.dart' show DebugTextRenderer;
+export 'src/debug_text_renderer.dart';
 export 'src/expect_color.dart';
 export 'src/expect_double.dart';
 export 'src/fails_assert.dart';