[WIP] Tests for stacked permuted dense class.

[dance858]

Work in progress. Do not merge.

[github-actions]

Benchmarks that have improved:

   before           after         ratio
 [934e3388]       [43021ea4]

•     1.16±0s          950±0ms     0.82  gini_portfolio.Cajas.time_compile_problem
  

Benchmarks that have stayed the same:

   before           after         ratio
 [934e3388]       [43021ea4]
     52.1±0ms         53.8±0ms     1.03  matrix_stuffing.SmallMatrixStuffing.time_compile_problem
      818±0ms          844±0ms     1.03  matrix_stuffing.ConeMatrixStuffingBench.time_compile_problem
     15.4±0ms         15.7±0ms     1.02  simple_QP_benchmarks.ParametrizedQPBenchmark.time_compile_problem
      292±0ms          298±0ms     1.02  matrix_stuffing.ParamSmallMatrixStuffing.time_compile_problem
      1.42±0s          1.45±0s     1.02  matrix_stuffing.ParamConeMatrixStuffing.time_compile_problem
      5.33±0s          5.43±0s     1.02  optimal_advertising.OptimalAdvertising.time_compile_problem
      4.04±0s          4.09±0s     1.01  huber_regression.HuberRegression.time_compile_problem
      12.8±0s          12.9±0s     1.01  finance.CVaRBenchmark.time_compile_problem
     24.3±0ms         24.4±0ms     1.00  high_dim_convex_plasticity.ConvexPlasticity.time_compile_problem
      1.05±0s          1.04±0s     1.00  finance.FactorCovarianceModel.time_compile_problem
      1.62±0s          1.61±0s     0.99  tv_inpainting.TvInpainting.time_compile_problem
      21.1±0s          20.9±0s     0.99  sdp_segfault_1132_benchmark.SDPSegfault1132Benchmark.time_compile_problem
      1.82±0s          1.80±0s     0.99  simple_QP_benchmarks.UnconstrainedQP.time_compile_problem
      3.25±0s          3.20±0s     0.99  quantum_hilbert_matrix.QuantumHilbertMatrix.time_compile_problem
      242±0ms          238±0ms     0.98  simple_QP_benchmarks.SimpleQPBenchmark.time_compile_problem
      4.64±0s          4.56±0s     0.98  svm_l1_regularization.SVMWithL1Regularization.time_compile_problem
      10.6±0s          10.4±0s     0.98  simple_LP_benchmarks.SimpleLPBenchmark.time_compile_problem
     14.8±0ms         14.4±0ms     0.98  simple_LP_benchmarks.SimpleFullyParametrizedLPBenchmark.time_compile_problem
      241±0ms          235±0ms     0.98  gini_portfolio.Murray.time_compile_problem
      295±0ms          288±0ms     0.97  slow_pruning_1668_benchmark.SlowPruningBenchmark.time_compile_problem
      936±0ms          904±0ms     0.97  simple_LP_benchmarks.SimpleScalarParametrizedLPBenchmark.time_compile_problem
      331±0ms          319±0ms     0.96  gini_portfolio.Yitzhaki.time_compile_problem
      786±0ms          757±0ms     0.96  simple_QP_benchmarks.LeastSquares.time_compile_problem
      546±0ms          507±0ms     0.93  semidefinite_programming.SemidefiniteProgramming.time_compile_problem