test-fileio: test read_virtual_file() with more files from /proc

test-fileio: test read_virtual_file() with more files from /proc

i.e. let's pick some files we know are too large, or where struct stat's
.st_size is zero even though non-empty, and test read_virtual_file()
with that, to ensure things are handled sensibly. Goal is to ensure all
three major codepaths in read_virtual_file() are tested.

Prompted-by: #20743