Auto merge of #7118 - g-k:tidy-check-unused-html-files, r=jdm

Add tidy check for unused reftest html files

Refs: https://github.com/servo/servo/issues/7078

Sample output:

```
$ time ./mach test-tidy
tests/ref/background_image_a.html not used or commented out in basic.list
tests/ref/background_image_ref.html not used or commented out in basic.list
tests/ref/canvas_linear_gradient_a.html not used or commented out in basic.list
tests/ref/canvas_linear_gradient_ref.html not used or commented out in basic.list
tests/ref/canvas_radial_gradient_a.html not used or commented out in basic.list
tests/ref/canvas_radial_gradient_ref.html not used or commented out in basic.list
tests/ref/inline_border_a.html not used or commented out in basic.list
tests/ref/inline_border_b.html not used or commented out in basic.list
tests/ref/inline_text_align_a.html not used or commented out in basic.list
tests/ref/inline_text_align_b.html not used or commented out in basic.list
tests/ref/link_style_dynamic_addition.html not used or commented out in basic.list
tests/ref/link_style_dynamic_addition_ref.html not used or commented out in basic.list
tests/ref/overflow_position_abs_inside_normal_a.html not used or commented out in basic.list
tests/ref/overflow_position_abs_inside_normal_b.html not used or commented out in basic.list
tests/ref/overflow_position_abs_simple_a.html not used or commented out in basic.list
tests/ref/overflow_position_abs_simple_b.html not used or commented out in basic.list
tests/ref/position_fixed_a.html not used or commented out in basic.list
tests/ref/position_fixed_b.html not used or commented out in basic.list
tests/ref/position_fixed_simple_a.html not used or commented out in basic.list
tests/ref/position_fixed_simple_b.html not used or commented out in basic.list
tests/ref/position_fixed_static_y_a.html not used or commented out in basic.list
tests/ref/position_fixed_static_y_b.html not used or commented out in basic.list
tests/ref/style_is_in_doc.html not used or commented out in basic.list
tests/ref/style_is_in_doc_ref.html not used or commented out in basic.list
tests/ref/table_specified_width_a.html not used or commented out in basic.list
tests/ref/table_specified_width_ref.html not used or commented out in basic.list
tests/ref/text_decoration_propagation_a.html not used or commented out in basic.list
tests/ref/text_decoration_propagation_b.html not used or commented out in basic.list
tests/ref/text_shadow_multiple_shadows_a.html not used or commented out in basic.list
tests/ref/text_shadow_multiple_shadows_ref.html not used or commented out in basic.list
tests/ref/viewport_percentage_vmin_vmax_b.html not used or commented out in basic.list
tests/ref/viewport_percentage_vw_vh_b.html not used or commented out in basic.list
tests/ref/white_space_intrinsic_sizes_a.html not used or commented out in basic.list
tests/ref/white_space_intrinsic_sizes_ref.html not used or commented out in basic.list
tests/ref/fonts/takao-p-gothic/COPYING.html not used or commented out in basic.list
tests/ref/iframe/multiple_external_child.html not used or commented out in basic.list
...
```

I thought it might be helpful to say which files aren't used in `basic.list`.

`./mach test-tidy` is a second or two slower on my laptop.
Not sure if this counts as a warning or an error that should return 1.
Not sure whether unused file output should go before or after the line specific errors.

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