tests: conftest: auto-add slow marker

[blueyed]

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[codecov]

Codecov Report

Merging #5341 into features will decrease coverage by 4.78%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           features    #5341      +/-   ##
============================================
- Coverage     93.59%   88.81%   -4.79%     
============================================
  Files           115      115              
  Lines         26307    26308       +1     
  Branches       2600     2600              
============================================
- Hits          24622    23365    -1257     
- Misses         1377     2610    +1233     
- Partials        308      333      +25

Impacted Files	Coverage Δ
testing/conftest.py	100% <100%> (ø)	⬆️
testing/test_reports.py	10.49% <0%> (-89.51%)	⬇️
testing/logging/test_reporting.py	22.06% <0%> (-77.94%)	⬇️
testing/examples/test_issue519.py	33.33% <0%> (-66.67%)	⬇️
testing/test_runner.py	34.7% <0%> (-61.61%)	⬇️
testing/test_runner_xunit.py	43.85% <0%> (-56.15%)	⬇️
testing/test_resultlog.py	53.6% <0%> (-46.4%)	⬇️
src/_pytest/reports.py	68.96% <0%> (-26.11%)	⬇️
testing/python/fixtures.py	76.05% <0%> (-22.46%)	⬇️
testing/test_warnings.py	26.77% <0%> (-21.86%)	⬇️
... and 20 more

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