TST: stats: add tests for all discrete distributions
TST: stats: add tests for all discrete distributions

Add tests for the cdf method of all the discrete distribution.

Stefan-Endres/scipy,Stefan-Endres/scipy,ilayn/scipy,zerothi/scipy,WarrenWeckesser/scipy,anntzer/scipy,endolith/scipy,scipy/scipy,e-q/scipy,andyfaff/scipy,perimosocordiae/scipy,e-q/scipy,anntzer/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,anntzer/scipy,grlee77/scipy,grlee77/scipy,matthew-brett/scipy,rgommers/scipy,endolith/scipy,grlee77/scipy,WarrenWeckesser/scipy,scipy/scipy,tylerjereddy/scipy,grlee77/scipy,scipy/scipy,Stefan-Endres/scipy,mdhaber/scipy,Eric89GXL/scipy,scipy/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,ilayn/scipy,e-q/scipy,grlee77/scipy,matthew-brett/scipy,andyfaff/scipy,andyfaff/scipy,Eric89GXL/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,vigna/scipy,vigna/scipy,endolith/scipy,endolith/scipy,scipy/scipy,andyfaff/scipy,tylerjereddy/scipy,matthew-brett/scipy,endolith/scipy,mdhaber/scipy,zerothi/scipy,Stefan-Endres/scipy,zerothi/scipy,zerothi/scipy,rgommers/scipy,mdhaber/scipy,tylerjereddy/scipy,perimosocordiae/scipy,andyfaff/scipy,mdhaber/scipy,matthew-brett/scipy,zerothi/scipy,scipy/scipy,anntzer/scipy,tylerjereddy/scipy,rgommers/scipy,Stefan-Endres/scipy,zerothi/scipy,matthew-brett/scipy,ilayn/scipy,vigna/scipy,Eric89GXL/scipy,perimosocordiae/scipy,endolith/scipy,Eric89GXL/scipy,tylerjereddy/scipy,perimosocordiae/scipy,mdhaber/scipy,anntzer/scipy,rgommers/scipy,rgommers/scipy,Eric89GXL/scipy,anntzer/scipy,vigna/scipy,e-q/scipy,WarrenWeckesser/scipy,ilayn/scipy,ilayn/scipy,vigna/scipy,WarrenWeckesser/scipy,e-q/scipy,mdhaber/scipy,andyfaff/scipy,ilayn/scipy