Should the debug screenshots be renamed from `*_.png` to `*_debug.png`

[schloerke]

cc @cpsievert

When traversing the test files, the trailing _ png files are a bit of a surprise. If it was a touch more verbose, then it would be clear as to the intent of the extra screenshot. Ex: NAME-DDD_debug.png vs (the current) NAME-DDD_.png.

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Notes:

• Would require vignette updates and updates to the git ignore suggestions / methods.
• Copy in the paragraph from robust.Rmd vignette on the debug screenshot into $expect_values() for more visibility on why the extra file exists.