volume-test: Allow rounding without having to allow a random number of errors

The rationale is that, as we allow rounding errors, instead of setting a number of times that will change over time and probably hide some other errors as it grows, allowing a + or - 1 difference in the deviation would allow us to avoid that guess game on the deviation and ignore the rounding issue we are having. Tested the build on a chroot on amd64 with gcc8. Works fine. Let me know what you guys think.

Fixes: #559 (closed) Closes: !4 (closed)