testing/conftest.py: Reintialize config._openfiles for each test
testing/conftest.py: Reintialize config._openfiles for each test

And no longer need getopenfiles or config._numfiles

--HG--
branch : refactor_LsofFdLeakChecker

mdboom/pytest,ericdill/pytest,malinoff/pytest,MengJueM/pytest,nicoddemus/repo-test,The-Compiler/pytest,ghostsquad/pytest,wfxiang08/pytest,chiller/pytest,omarkohl/pytest,ojake/pytest,oleg-alexandrov/pytest,ionelmc/pytest,inirudebwoy/pytest,ropez/pytest,pelme/pytest,vmalloc/dessert,hpk42/pytest,icemac/pytest,chillbear/pytest,bubenkoff/pytest,pfctdayelise/pytest,takluyver/pytest,jb098/pytest,hunse/pytest,bukzor/pytest,hpk42/pytest,abusalimov/pytest,omarkohl/pytest,Bachmann1234/pytest,hackebrot/pytest,MengJueM/pytest,tomviner/pytest,ropez/pytest,mhils/pytest,bukzor/pytest,Haibo-Wang-ORG/pytest,ionelmc/pytest,jb098/pytest,ericdill/pytest,jaraco/pytest,pytest-dev/pytest,tomviner/pytest,ghostsquad/pytest,untitaker/pytest,rouge8/pytest,ddboline/pytest,rouge8/pytest,mbirtwell/pytest,skylarjhdownes/pytest,gabrielcnr/pytest,JonathonSonesen/pytest,vodik/pytest,mhils/pytest,mdboom/pytest,RonnyPfannschmidt/pytest,chiller/pytest,davidszotten/pytest,tgoodlet/pytest,Carreau/pytest,codewarrior0/pytest,codewarrior0/pytest,tareqalayan/pytest,ojake/pytest,etataurov/pytest,alfredodeza/pytest,chillbear/pytest,abusalimov/pytest,nicoddemus/pytest,oleg-alexandrov/pytest,doordash/pytest,The-Compiler/pytest,nicoddemus/pytest,doordash/pytest,userzimmermann/pytest,vodik/pytest,Bjwebb/pytest,takluyver/pytest,txomon/pytest,nicoddemus/repo-test,bubenkoff/pytest,gabrielcnr/pytest,flub/pytest,lukas-bednar/pytest,Carreau/pytest,lukas-bednar/pytest,eli-b/pytest,Bachmann1234/pytest,userzimmermann/pytest,Akasurde/pytest,wfxiang08/pytest,JonathonSonesen/pytest,mbirtwell/pytest,markshao/pytest,rmfitzpatrick/pytest,pelme/pytest,Haibo-Wang-ORG/pytest,hunse/pytest,Bjwebb/pytest,MichaelAquilina/pytest,icemac/pytest,inirudebwoy/pytest,untitaker/pytest