FYI. After installing the update and content (including the test pilot livery) I could not see out the windows of the c172 Classic. The windows were grey and opaque.
I deleted the c172 Classic folders in the Official folder and allowed them to be reinstalled by the sim, and the problem remained.
Luckily I had another un-updated copy on a 2nd machine so I grabbed the Classic files from there and put them on my main machine.
MSFS still updated them, but I didn’t update the Testpilot livery. Whether this had any relationship to the problem, I don’t know, but the problem is resolved.
I kept a copy of the corrupted files and there are some definite differences in file sizes compared to what was updated after I restored the older files.
Edit: To reconfirm my findings, I deleted the c172 Classic folders again and allowed MSFS to reinstall them. Using a folder compare program, I found that the update didn’t install the windshield or seat belt textures along with some other files having a different filesize.
I have raised a ZenDesk report