[WIP] Tests for stacked permuted dense class.

[dance858]

Work in progress. Do not merge.

[github-actions]

Benchmarks that have stayed the same:

   before           after         ratio
 [934e3388]       [035eda97]
     52.8±0ms         54.5±0ms     1.03  matrix_stuffing.SmallMatrixStuffing.time_compile_problem
      969±0ms          985±0ms     1.02  gini_portfolio.Cajas.time_compile_problem
      307±0ms          309±0ms     1.01  slow_pruning_1668_benchmark.SlowPruningBenchmark.time_compile_problem
      5.42±0s          5.45±0s     1.01  optimal_advertising.OptimalAdvertising.time_compile_problem
      1.83±0s          1.83±0s     1.00  simple_QP_benchmarks.UnconstrainedQP.time_compile_problem
      13.9±0s          13.9±0s     1.00  finance.CVaRBenchmark.time_compile_problem
      243±0ms          243±0ms     1.00  simple_QP_benchmarks.SimpleQPBenchmark.time_compile_problem
      4.09±0s          4.09±0s     1.00  huber_regression.HuberRegression.time_compile_problem
      1.43±0s          1.42±0s     1.00  matrix_stuffing.ParamConeMatrixStuffing.time_compile_problem
      823±0ms          820±0ms     1.00  matrix_stuffing.ConeMatrixStuffingBench.time_compile_problem
      775±0ms          772±0ms     1.00  simple_QP_benchmarks.LeastSquares.time_compile_problem
      1.65±0s          1.64±0s     1.00  tv_inpainting.TvInpainting.time_compile_problem
      296±0ms          295±0ms     0.99  matrix_stuffing.ParamSmallMatrixStuffing.time_compile_problem
      4.76±0s          4.72±0s     0.99  svm_l1_regularization.SVMWithL1Regularization.time_compile_problem
      21.2±0s          20.9±0s     0.99  sdp_segfault_1132_benchmark.SDPSegfault1132Benchmark.time_compile_problem
      239±0ms          236±0ms     0.99  gini_portfolio.Murray.time_compile_problem
      1.07±0s          1.05±0s     0.98  finance.FactorCovarianceModel.time_compile_problem
      10.8±0s          10.6±0s     0.98  simple_LP_benchmarks.SimpleLPBenchmark.time_compile_problem
      540±0ms          529±0ms     0.98  semidefinite_programming.SemidefiniteProgramming.time_compile_problem
     16.1±0ms         15.6±0ms     0.97  simple_QP_benchmarks.ParametrizedQPBenchmark.time_compile_problem
     26.3±0ms         25.3±0ms     0.96  high_dim_convex_plasticity.ConvexPlasticity.time_compile_problem
      955±0ms          918±0ms     0.96  simple_LP_benchmarks.SimpleScalarParametrizedLPBenchmark.time_compile_problem
      347±0ms          329±0ms     0.95  gini_portfolio.Yitzhaki.time_compile_problem
     15.2±0ms         14.4±0ms     0.95  simple_LP_benchmarks.SimpleFullyParametrizedLPBenchmark.time_compile_problem
      3.33±0s          3.14±0s     0.94  quantum_hilbert_matrix.QuantumHilbertMatrix.time_compile_problem