Skip tests generated by plugins (like pep8) for discovery command.

[zztalker]

To do not fail on discovery command and do not track their results.

For #7287

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Appropriate comments and documentation strings in the code
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)
• The wiki is updated with any design decisions/details.

[msftclas]

All CLA requirements met.

[codecov-io]

Codecov Report

Merging #8242 into master will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8242      +/-   ##
==========================================
+ Coverage   59.04%   59.37%   +0.32%     
==========================================
  Files         509      509              
  Lines       23299    23299              
  Branches     3758     3763       +5     
==========================================
+ Hits        13758    13833      +75     
+ Misses       8642     8567      -75     
  Partials      899      899

Impacted Files	Coverage Δ
src/client/language/characters.ts	33.33% <0%> (+2.56%)	⬆️
src/client/common/utils/version.ts	81.48% <0%> (+3.7%)	⬆️
src/client/providers/providerUtilities.ts	88.88% <0%> (+5.55%)	⬆️
src/client/datascience/cellFactory.ts	70.51% <0%> (+12.82%)	⬆️
src/client/api.ts	92.85% <0%> (+14.28%)	⬆️
src/client/common/utils/icons.ts	100% <0%> (+16.66%)	⬆️
...cience/jupyter/jupyterDebuggerNotInstalledError.ts	80% <0%> (+20%)	⬆️
src/client/formatters/serviceRegistry.ts	100% <0%> (+25%)	⬆️
...ience/jupyter/jupyterDebuggerRemoteNotSupported.ts	100% <0%> (+25%)	⬆️
...t/datascience/jupyter/jupyterDataRateLimitError.ts	100% <0%> (+33.33%)	⬆️
... and 2 more

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[karthiknadig]

The changes look good, might need a rebase to master to get around the test failures.

[ericsnowcurrently]

LGTM (with a mild caution)

[ericsnowcurrently]

FWIW, I am fairly sure that the old code (parsing the pytest CLI output) used to ignore plugin-generated nodes. If it matters then we can investigate in #8425.