Fix for ex_unit diff bug

[dkuku]

I found the reason of #14268
we passed the length of left list and then the code looked like this:

left = [:b, :c]
right = [:a, :b, :c]
{parsed_right = [:a, :b], improper_right = [:c]} = split_right_list(right, length(left), [])

I added one test, and fixed two one, which looks like it's invalid currently.
There are still two failing tests - I may need a hand with this.

[dkuku]

It's ready

[josevalim]

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