[WIP] Pytester: disable plugin autoload

[blueyed]

Includes / based on #4902.

Makes it easier to use pytest-randomly etc with pytest's tests itself (#4351).

[codecov]

Codecov Report

Merging #5050 into features will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           features    #5050      +/-   ##
============================================
+ Coverage     96.07%   96.08%   +0.01%     
============================================
  Files           114      114              
  Lines         25778    25816      +38     
  Branches       2552     2556       +4     
============================================
+ Hits          24765    24805      +40     
- Misses          704      705       +1     
+ Partials        309      306       -3

Impacted Files	Coverage Δ
testing/test_assertion.py	97.62% <100%> (ø)	⬆️
testing/test_pytester.py	89.14% <100%> (+0.85%)	⬆️
testing/acceptance_test.py	98.03% <100%> (ø)	⬆️
testing/test_terminal.py	99.82% <100%> (ø)	⬆️
src/_pytest/pytester.py	91.28% <100%> (+0.11%)	⬆️
testing/test_helpconfig.py	97.36% <100%> (+0.07%)	⬆️
src/_pytest/terminal.py	92.62% <0%> (+0.16%)	⬆️
src/_pytest/helpconfig.py	98.19% <0%> (+0.9%)	⬆️

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[blueyed]

Duplicate of #4518.