test-stat-util: don't fail under chroot

I wanted to see what is_path_read_only_fs() and is_path_temporary_fs() return
in a chroot, and various tests would fail. For most of our codebase, we can
assume that /proc and such are mounted, and it doesn't make sense to make the
tests work in a chroot. But let's do it here. (In general, it would be useful
for most stuff in src/basic/, since it's linked into libraries which might be
invoked in incorrectly set up environments and should not fail too badly.)