Fix test failures after pluggy 1.8 release

[nicoddemus]

pluggy now calls iter_entry_points with different arguments, and tests
which mocked that call need to be updated accordingly.

[codecov]

Codecov Report

Merging #4816 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4816      +/-   ##
==========================================
+ Coverage   95.61%   95.66%   +0.04%     
==========================================
  Files         113      113              
  Lines       25163    25163              
  Branches     2498     2498              
==========================================
+ Hits        24060    24071      +11     
+ Misses        776      772       -4     
+ Partials      327      320       -7

Flag	Coverage Δ
#docs	29.59% <0%> (+0.1%)	⬆️
#doctesting	29.59% <0%> (+0.1%)	⬆️
#linting	29.59% <0%> (+0.1%)	⬆️
#linux	95.49% <100%> (+0.03%)	⬆️
#nobyte	92.2% <100%> (ø)	⬆️
#numpy	93% <100%> (+0.03%)	⬆️
#pexpect	41.98% <0%> (+0.02%)	⬆️
#pluggymaster	93.72% <100%> (?)
#py27	93.68% <100%> (+0.04%)	⬆️
#py34	91.59% <100%> (ø)	⬆️
#py35	91.61% <100%> (-0.01%)	⬇️
#py36	91.64% <100%> (ø)	⬆️
#py37	93.82% <100%> (+0.03%)	⬆️
#trial	93% <100%> (+0.03%)	⬆️
#windows	93.67% <100%> (+0.1%)	⬆️
#xdist	93.7% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
testing/test_assertion.py	97.59% <ø> (ø)	⬆️
testing/test_config.py	99.63% <100%> (ø)	⬆️
src/_pytest/pytester.py	87.05% <0%> (+0.42%)	⬆️
src/_pytest/terminal.py	91.74% <0%> (+0.82%)	⬆️
src/_pytest/cacheprovider.py	97.16% <0%> (+1.41%)	⬆️

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