Port truncate tests to C++

[terasakisatoshi]

@testset "truncate" begin
        sve = SparseIR.CentrosymmSVE(LogisticKernel(5), 1e-6, Float64)

        svds = SparseIR.compute_svd.(SparseIR.matrices(sve))
        u_, s_, v_ = zip(svds...)

        for lmax in 3:20
            u, s, v = SparseIR.truncate(u_, s_, v_; lmax)
            u, s, v = SparseIR.postprocess(sve, u, s, v)
            @test length(u) == length(s) == length(v)
            @test length(s) ≤ lmax - 1
        end
    end

[terasakisatoshi]

#46 was not sufficient.

[terasakisatoshi]

Need to write the truncate function more seriously.

[terasakisatoshi]

#51 fixed