util/flow-rate: fix sum calc on index next to base

util/flow-rate: fix sum calc on index next to base

When the buffer is wrapped around, for any new index, the calculation
must subtract the previous value stored in the buffer. So far, the code
ended up adding to the existing buffer value on the index unless it was
the first index after wrapping around. This is incorrect and would end
up flagging a flow as elephant a lot before than it should be.

Harden the Test06 by checking for such a case.

Bug 7694