use identity checks for the mock sentinels

[RonnyPfannschmidt]

fixes #5606

[codecov]

Codecov Report

Merging #5631 into master will increase coverage by 1.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5631      +/-   ##
==========================================
+ Coverage   94.76%   96.12%   +1.35%     
==========================================
  Files         117      117              
  Lines       25770    25773       +3     
  Branches     2495     2493       -2     
==========================================
+ Hits        24422    24774     +352     
+ Misses       1030      695     -335     
+ Partials      318      304      -14

Impacted Files	Coverage Δ
testing/python/integration.py	94.28% <100%> (+0.21%)	⬆️
src/_pytest/compat.py	98.5% <100%> (+1.44%)	⬆️
src/_pytest/pastebin.py	87.71% <0%> (-3.51%)	⬇️
testing/test_pytester.py	89.27% <0%> (+0.57%)	⬆️
src/_pytest/pytester.py	90.96% <0%> (+1.03%)	⬆️
src/_pytest/capture.py	94.75% <0%> (+1.31%)	⬆️
testing/test_terminal.py	99.84% <0%> (+1.4%)	⬆️
src/_pytest/terminal.py	93.02% <0%> (+2.27%)	⬆️
src/_pytest/unittest.py	94.97% <0%> (+5.58%)	⬆️
src/_pytest/debugging.py	90.05% <0%> (+20.41%)	⬆️
... and 1 more

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[blueyed]

testdir.syspathinsert is not necessary here (tested without it).

[blueyed]

Please squash.