simulate file IOError with fs-verity

[ThomasWaldmann]

to check borg's I/O error behaviour, we could try adding this on linux:

• create a file
• seal it with fs-verity
• flip a bit in the middle
• get a read error when trying to read that place

https://www.kernel.org/doc/html/latest/filesystems/fsverity.html

[ThomasWaldmann]

Might be useful to test the fix for #6709.