TST: stats: add kstwo, ksone to slow tests.

[rgommers]

This speeds up stats.test() by almost 50% (60 sec to 31 sec)
when running locally. The kstwo test is also the single slowest test
on CI, see gh-12132.

Output before this commit:

========================================================= slowest 40 test durations =========================================================
17.48s call     tests/test_continuous_basic.py::test_cont_basic[kstwo-arg59]
5.02s call     tests/test_stats.py::TestMGCStat::test_random_state
3.46s call     tests/test_stats.py::TestMGCStat::test_workers
3.17s call     tests/test_continuous_basic.py::test_cont_basic[ksone-arg58]
1.84s call     tests/test_stats.py::TestKSTwoSamples::testMediumBoth
1.71s call     tests/test_stats.py::TestKSTwoSamples::testMiddlingBoth
1.34s call     tests/test_continuous_basic.py::test_cont_basic[norminvgauss-arg78]
1.23s call     tests/test_mstats_basic.py::TestCorr::test_kendalltau
1.03s call     tests/test_stats.py::TestKSTwoSamples::test_some_code_paths
0.79s call     tests/test_continuous_basic.py::test_cont_basic[ncx2-arg76]
0.72s call     tests/test_continuous_basic.py::test_cont_basic[geninvgauss-arg35]
0.59s call     tests/test_continuous_basic.py::test_cont_basic[skewnorm-arg90]
0.50s call     tests/test_continuous_basic.py::test_cont_basic[burr-arg7]
0.47s call     tests/test_continuous_basic.py::test_cont_basic[exponweib-arg20]
0.42s call     tests/test_continuous_basic.py::test_cont_basic[tukeylambda-arg97]
0.36s call     tests/test_continuous_basic.py::test_cont_basic[pearson3-arg80]
0.33s call     tests/test_mstats_basic.py::TestCompareWithStats::test_kendalltau
0.33s call     tests/test_continuous_basic.py::test_cont_basic[johnsonsb-arg51]
0.31s call     tests/test_continuous_basic.py::test_cont_basic[powernorm-arg83]
0.29s call     tests/test_continuous_basic.py::test_cont_basic[exponpow-arg19]
0.29s call     tests/test_continuous_basic.py::test_cont_basic[crystalball-arg13]
0.27s call     tests/test_continuous_basic.py::test_cont_basic[beta-arg4]
0.27s call     tests/test_continuous_basic.py::test_cont_basic[triang-arg93]
0.26s call     tests/test_continuous_basic.py::test_cont_basic[exponnorm-arg18]
0.26s call     tests/test_continuous_basic.py::test_cont_basic[fatiguelife-arg22]
0.25s call     tests/test_continuous_basic.py::test_cont_basic[burr12-arg8]
0.25s call     tests/test_continuous_basic.py::test_cont_basic[gengamma-arg33]
0.25s call     tests/test_continuous_basic.py::test_cont_basic[betaprime-arg5]
0.23s call     tests/test_continuous_basic.py::test_cont_basic[halfgennorm-arg38]
0.22s call     tests/test_continuous_basic.py::test_cont_basic[genhalflogistic-arg34]
0.22s call     tests/test_continuous_basic.py::test_cont_basic[f-arg21]
0.19s call     tests/test_stats.py::TestFOneWay::test_nist
0.19s call     tests/test_continuous_basic.py::test_cont_basic[chi-arg10]
0.19s call     tests/test_continuous_basic.py::test_cont_basic[invgauss-arg49]
0.19s call     tests/test_continuous_basic.py::test_cont_basic[nct-arg75]
0.18s call     tests/test_continuous_basic.py::test_cont_basic[genpareto-arg39]
0.18s call     tests/test_multivariate.py::TestMultivariateNormal::test_broadcasting
0.18s call     tests/test_distributions.py::TestLevyStable::test_pdf_alpha_equals_one_beta_non_zero
0.16s call     tests/test_distributions.py::TestSkewNorm::test_moments
0.16s call     tests/test_continuous_basic.py::test_cont_basic[gompertz-arg41]

I'm leaving TestMGCStat::test_workers as the only test that takes >1.5 sec, to not make test coverage for MGC too low.

[rgommers]

@pvanmulbregt I think some of these performance regressions for kstwo were expected and necessary because of accuracy fixes. Can't remember though, would be good if you could have a look.

[pvanmulbregt]

kstwo is a new distribution, for better accuracy than kstwobign.
The _cdf and sf implementations seem reasonable.
The _pdf() method is implemented as the derivative of the CDF, and the ppf uses optimize.brent to find the root.
The moments are calculated via integration.
The upshot is that the entropy and stats calls in the test_cont_basic test take quite some time to evaluate. Switching from momtype=0 to momtype=1 made very little difference.

[larsoner]

Seems like a useful gain in speed to me

[rgommers]

Thanks @pvanmulbregt. I figured it was something like that. accuracy > speed, so let's leave it as is for now. Just wanted to make sure you saw I moved these to @slow.

Thanks for the approval @larsoner . Merging then.