Fixed failing tests due to range error

[bmuddha]

The issue was that helper account method generated AccountData
values with len() ranging from 1, in case if len turned out to be 1,
then valid_ranges couldn't generate range, since it also tried to
build range starting from value 1 and ending with whatever is the len() of
AccountData, 1..1 in case which triggered error.

As a solution, the minimum AccountData length was set to 2, since
there's no logic requirements for it to start from 1.

Closes #159

[00nktk]

Yeah, that fixes the issue, but we still want to test 1 byte accounts since that can be a tricky corner case.
I believe we can filter out empty ranges the same way it's done for offsets (see fn offset on 43, it returns one-item strategy for a 1 byte account).

I think the best way is to refactor offsets function so we can use it to generate strategies for both offsets and slice lengths.

[bmuddha]

I think the best way is to refactor offsets function so we can use it to generate strategies for both offsets and slice lengths.

I've rewrote the function to generate both offsets and slice length, with special handling of AccountData::len() being 1, will that do?