Rename clashing variables in test

[schillic]

This was causing the following warnings:

┌ Warning: Assignment to `SSPZ` in soft scope is ambiguous because a global variable by the same name exists: `SSPZ` will be treated as a new local. Disambiguate by using `local SSPZ` to suppress this warning or `global SSPZ` to assign to the existing global variable.
└ @ ~/work/LazySets.jl/LazySets.jl/test/Sets/SparsePolynomialZonotope.jl:109
┌ Warning: Assignment to `SPZ` in soft scope is ambiguous because a global variable by the same name exists: `SPZ` will be treated as a new local. Disambiguate by using `local SPZ` to suppress this warning or `global SPZ` to assign to the existing global variable.
└ @ ~/work/LazySets.jl/LazySets.jl/test/Sets/SparsePolynomialZonotope.jl:110