Fix painting API examples tests directories structure. (#148177)

This PR contributes to https://github.com/flutter/flutter/issues/130459

### Description
- Fixes directories structure for the existing painting API examples tests

dev/bots/check_code_samples.dart

@@ -365,11 +365,7 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0_test.dart',
   'examples/api/test/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.1_test.dart',
   'examples/api/test/rendering/scroll_direction/scroll_direction.0_test.dart',
-  'examples/api/test/painting/axis_direction/axis_direction.0_test.dart',
-  'examples/api/test/painting/linear_border/linear_border.0_test.dart',
-  'examples/api/test/painting/gradient/linear_gradient.0_test.dart',
   'examples/api/test/painting/star_border/star_border.0_test.dart',
-  'examples/api/test/painting/borders/border_side.stroke_align.0_test.dart',
   'examples/api/test/widgets/autocomplete/raw_autocomplete.focus_node.0_test.dart',
   'examples/api/test/widgets/autocomplete/raw_autocomplete.2_test.dart',
   'examples/api/test/widgets/autocomplete/raw_autocomplete.1_test.dart',