Relax INEXACT_ROUND conformance

Update the bounds calculations to account for floating point rounding
when summing the output zero points in rescale.

Also add some unit tests for inexact rescale in the output_unsigned case
because the "signless" input value could easily be interpreted
incorrectly when converting to floating point.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ia2b1fba90162b78772ce15c2f4fc3a521967b4b4