handle race condition when creation and deletion of a numbered dir overlap

[RonnyPfannschmidt]

• handle test folder cleanup being unable to create a cleanup lock

for the deferred elements i created issues

[codecov]

Codecov Report

Merging #4222 into master will increase coverage by 0.97%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4222      +/-   ##
==========================================
+ Coverage   95.64%   96.62%   +0.97%     
==========================================
  Files         109      109              
  Lines       24597    34281    +9684     
  Branches     2394     4379    +1985     
==========================================
+ Hits        23527    33124    +9597     
- Misses        759      892     +133     
+ Partials      311      265      -46

Flag	Coverage Δ
#docs	28.49% <7.69%> (?)
#doctesting	28.49% <7.69%> (?)
#linting	28.49% <7.69%> (?)
#linux	96.29% <100%> (+0.64%)	⬆️
#nobyte	93.36% <100%> (+1.98%)	⬆️
#numpy	78.81% <76.47%> (+37.14%)	⬆️
#pexpect	41.67% <23.07%> (-0.01%)	⬇️
#py27	95.07% <100%> (+1.14%)	⬆️
#py34	92.23% <100%> (+0.05%)	⬆️
#py35	93.69% <100%> (+1.49%)	⬆️
#py36	95.05% <100%> (+1.23%)	⬆️
#py37	92.36% <100%> (+0.04%)	⬆️
#trial	78.81% <76.47%> (+37.14%)	⬆️
#windows	94.04% <100%> (?)
#xdist	95.01% <100%> (+1.27%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/pathlib.py	93.67% <100%> (+6.88%)	⬆️
testing/test_tmpdir.py	97.9% <100%> (+3.79%)	⬆️
testing/test_pdb.py	70.66% <0%> (-24.34%)	⬇️
src/_pytest/debugging.py	86.12% <0%> (-8%)	⬇️
testing/test_modimport.py	75% <0%> (-5%)	⬇️
src/_pytest/freeze_support.py	24.13% <0%> (-0.87%)	⬇️
testing/test_terminal.py	99.51% <0%> (-0.49%)	⬇️
src/_pytest/unittest.py	93.96% <0%> (-0.39%)	⬇️
testing/python/raises.py	92.75% <0%> (-0.25%)	⬇️
testing/test_skipping.py	98.46% <0%> (-0.22%)	⬇️
... and 86 more

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[nicoddemus]

Thanks @RonnyPfannschmidt!