tests: restore tracing function

[blueyed]

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

https://bugs.python.org/issue37011

[codecov]

Codecov Report

Merging #5406 into master will increase coverage by 2.21%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #5406      +/-   ##
==========================================
+ Coverage   93.76%   95.98%   +2.21%     
==========================================
  Files         114      114              
  Lines       25489    25496       +7     
  Branches     2472     2474       +2     
==========================================
+ Hits        23901    24473     +572     
+ Misses       1275      718     -557     
+ Partials      313      305       -8

Impacted Files	Coverage Δ
testing/conftest.py	90.32% <57.14%> (-9.68%)	⬇️
src/_pytest/config/__init__.py	94.33% <0%> (+0.64%)	⬆️
testing/test_pdb.py	99% <0%> (+0.79%)	⬆️
src/_pytest/python.py	93.03% <0%> (+0.85%)	⬆️
src/_pytest/pytester.py	91.02% <0%> (+0.88%)	⬆️
src/_pytest/terminal.py	93.06% <0%> (+1.41%)	⬆️
src/_pytest/tmpdir.py	100% <0%> (+3.7%)	⬆️
testing/python/fixtures.py	99.09% <0%> (+8.4%)	⬆️
testing/test_terminal.py	99.84% <0%> (+8.45%)	⬆️
testing/python/metafunc.py	94.88% <0%> (+18.73%)	⬆️
... and 4 more

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[blueyed]

Going to merge this as base for #5403 already.

[blueyed]

Backport: #5408