Optimize/revisit determine_setup

[blueyed]

Taken out of #4337.

[blueyed]

Note: only the "is None" branch appears to be covered here: https://codecov.io/gh/pytest-dev/pytest/commit/247bdb2c0f5ba64161de7f24e958919dfe0e02ea#D1-124

[blueyed]

Build failure is unrelated (test_fail_and_continue_with_stepwise), #4366.

[codecov]

Codecov Report

Merging #4367 into features will decrease coverage by 0.05%.
The diff coverage is 93.33%.

@@             Coverage Diff              @@
##           features    #4367      +/-   ##
============================================
- Coverage     95.85%   95.79%   -0.06%     
============================================
  Files           111      111              
  Lines         24950    25233     +283     
  Branches       2439     2486      +47     
============================================
+ Hits          23915    24172     +257     
- Misses          737      765      +28     
+ Partials        298      296       -2

Flag	Coverage Δ
#docs	29.13% <33.33%> (-0.01%)	⬇️
#doctesting	29.13% <33.33%> (-0.01%)	⬇️
#linting	29.13% <33.33%> (-0.01%)	⬇️
#linux	95.62% <93.33%> (-0.02%)	⬇️
#nobyte	91.85% <93.33%> (-0.01%)	⬇️
#numpy	94.93% <93.33%> (+1.89%)	⬆️
#pexpect	40.79% <46.66%> (-0.84%)	⬇️
#py27	92.98% <93.33%> (-1.06%)	⬇️
#py34	92.16% <93.33%> (+0.06%)	⬆️
#py35	92.05% <93.33%> (-0.07%)	⬇️
#py36	93.89% <93.33%> (-0.02%)	⬇️
#py37	92.22% <93.33%> (+0.06%)	⬆️
#trial	94.93% <93.33%> (+1.89%)	⬆️
#windows	93.94% <93.33%> (-0.04%)	⬇️
#xdist	93.84% <93.33%> (+0.1%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/config/__init__.py	95.01% <100%> (ø)	⬆️
src/_pytest/config/findpaths.py	94.11% <92.85%> (-0.73%)	⬇️
testing/test_argcomplete.py	59.21% <0%> (-7.46%)	⬇️
testing/test_collection.py	97.63% <0%> (-2.15%)	⬇️
src/_pytest/unittest.py	93.22% <0%> (-1.13%)	⬇️
testing/test_nodes.py	100% <0%> (ø)	⬆️
testing/test_session.py	96.59% <0%> (+0.09%)	⬆️
src/_pytest/main.py	97.4% <0%> (+0.43%)	⬆️

---

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[RonnyPfannschmidt]

at initial skimming this looks correct and the test results indicate it is
good job at aking that one a bit quicker - could you post the difference it makes before we merge

[blueyed]

This was mostly for readability / saneness IIRC.

the test results indicate it is

Well, it is not 100% covered and we had things already where tests were passing, but they broke things then.

But since this is for features and relatively safe, I'm going to merge this already.