pdb: only use outcomes.exit via do_quit

[blueyed]

Fixes #5235.

[asottile]

[codecov]

Codecov Report

Merging #5236 into features will decrease coverage by 1.49%.
The diff coverage is 77.77%.

@@             Coverage Diff             @@
##           features    #5236     +/-   ##
===========================================
- Coverage     96.14%   94.65%   -1.5%     
===========================================
  Files           115      115             
  Lines         26125    26139     +14     
  Branches       2577     2578      +1     
===========================================
- Hits          25118    24741    -377     
- Misses          706     1083    +377     
- Partials        301      315     +14

Impacted Files	Coverage Δ
src/_pytest/debugging.py	88.82% <60%> (-0.36%)	⬇️
testing/test_pdb.py	98.82% <84.61%> (-0.38%)	⬇️
testing/test_pluginmanager.py	49.57% <0%> (-50.43%)	⬇️
testing/test_pytester.py	54.04% <0%> (-35.27%)	⬇️
testing/test_terminal.py	86.78% <0%> (-13.07%)	⬇️
src/_pytest/pytester.py	86.8% <0%> (-4.83%)	⬇️
src/_pytest/terminal.py	92.02% <0%> (-1.4%)	⬇️
testing/test_recwarn.py	98.3% <0%> (-1.28%)	⬇️
testing/test_runner.py	95.87% <0%> (-1.09%)	⬇️
src/_pytest/config/__init__.py	93.79% <0%> (-0.64%)	⬇️
... and 2 more

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