TST: [stats, discrete] tighten cdf-ppf tests
TST: [stats, discrete] tighten cdf-pmf tests
TST: [stats, discrete] remove non-running tests
TST: [stats, discrete] remove sample tests

They are too noisy: only work for decimal=0.
TST: [stats, discrete] tighten isf/ppf tests
Merge pull request #3138 from ev-br/discr_tests

clean up tests for discrete distributions