fix two tests in preparation for julia nightly

[benlorenz]

To adapt for changes in julia nightly, found while trying the Oscar tests locally with custom builds for the CxxWrap based packages.

Older julia versions have the weird effect of resizing arrays even when append! fails:

julia> v = [1,2,3];

julia> append!(v,[1.2]);
ERROR: InexactError: Int64(1.2)
...

julia> v
4-element Vector{Int64}:
 1
 2
 3
 0

This was changed in JuliaLang/julia#51903.

And julia might not always consider objects in the same scope as unused, even if they do not appear to be used anymore, see JuliaLang/julia#51818. This seems to depend where the GC marking is happening in the generated code and this was changed recently.
To avoid this one can put the code with the data that needs to be collected in a separate function (this pattern is also used in the julia tests).