Add more test to `RawBinarySignalRawIO`

[h-mayorquin]

Maybe we should add to the todo to add a test that catches this and test other aspects of reading non-int16 binary data with this (say unsigned types) to see if we are not making any assumptions that we should not.

Originally posted by @h-mayorquin in https://github.com/NeuralEnsemble/python-neo/issues/1765#issuecomment-3266620349