Google has a said that when the Google Structured Data Testing tool accesses a site, it uses a neutral crawler in terms of desktop versus mobile. It depends on what the site serves by default and John Mueller said, it probably will serve Google the desktop version.
Ashley Berman Hale asked what is the useragent for the Google Structured Data Testing Tool and John said it is neutral:

It’s neutral in that regard. Since it doesn’t explicitly mention mobile, I imagine most setups would treat it as desktop.
— 🍌 John 🍌 (@JohnMu) October 31, 2018
I do believe the useragent looks like this:

Mozilla/5.0 (compatible; Google-Structured-Data-Testing-Tool +https://search.google.com/structured-data/testing-tool)

If your site is responsive, you do not need to think about this much.
Forum discussion at Twitter.

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