Fix allocation tests

[dpo]

Some operators incur allocations in the unit tests:

https://github.com/JuliaSmoothOptimizers/ShiftedProximalOperators.jl/actions/runs/18427091179/job/52524497744?pr=144#step:6:297

[MaxenceGollier]

See JuliaLang/julia#57064.

Edit: Testing for 16 allocations is very strange, actually the latest release of Julia fixed the 16 bytes.