[4.6] tests: restore tracing function

[blueyed]

Without this, testing/test_pdb.py (already without pexpect) will cause
missing test coverage afterwards (for the same process).

[codecov]

Codecov Report

Merging #5408 into 4.6-maintenance will increase coverage by 1.74%.
The diff coverage is 57.14%.

@@                 Coverage Diff                 @@
##           4.6-maintenance    #5408      +/-   ##
===================================================
+ Coverage            94.43%   96.18%   +1.74%     
===================================================
  Files                  115      115              
  Lines                26384    26391       +7     
  Branches              2603     2605       +2     
===================================================
+ Hits                 24916    25383     +467     
+ Misses                1143      703     -440     
+ Partials               325      305      -20

Impacted Files	Coverage Δ
testing/conftest.py	90.32% <57.14%> (-9.68%)	⬇️
src/_pytest/main.py	96.28% <0%> (+0.21%)	⬆️
src/_pytest/fixtures.py	97.55% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.25% <0%> (+0.29%)	⬆️
testing/test_terminal.py	99.84% <0%> (+0.31%)	⬆️
testing/test_config.py	99.83% <0%> (+0.33%)	⬆️
testing/python/collect.py	99.36% <0%> (+0.42%)	⬆️
src/_pytest/assertion/rewrite.py	95.77% <0%> (+0.48%)	⬆️
testing/python/fixtures.py	99.09% <0%> (+0.56%)	⬆️
src/_pytest/junitxml.py	95.9% <0%> (+0.58%)	⬆️
... and 25 more

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[asottile]

hmmm I would have put this in the function and orig_trace is not None

[blueyed]

Well, this way it avoids using the fixture when not run with coverage (i.e. no initial tracing).
(not sure I understand your suggestion though)

[nicoddemus]

I agree it seems better to not have this active without need.

[nicoddemus]

I agree it seems better to not have this active without need.

[nicoddemus]

I will go ahead and merge this as the original PR is already on master. 👍

[blueyed]

I will go ahead and merge this as the original PR is already on master. 

Thanks. We can still improve/change it though (possibly without a backport), if @asottile would like to have it.

[nicoddemus]

We can still improve/change it though (possibly without a backport), if @asottile would like to have it.

Definitely!

I just went ahead with the merge because IMO we should try to make the backports as close to the original as possible. Improvements suggested during the backport PR should happen in another round of PRs if they apply to the original patch.