Fix the "Problem" messages for pytest failures when debugging.

[ericsnowcurrently]

fixes #14866

In #14311 we worked around a bug in a way that caused the strange message reported in #14866 (pytest with debugger). This change fixes that one without regressing the previous fix.

[kimadeline]

The fix looks good to me. Is there a reason why we need verbose explanations for it in comments?

[karrtikr]

Please update existing tests if any. LGTM otherwise.

[kimadeline]

Did you check that your new changes are not re-introducing #14067?

[ericsnowcurrently]

Did you check that your new changes are not re-introducing #14067?

That was resolved with the changes in #15387. Test results with an undefined status will never reach the problematic code now. Furthermore, with this PR, diagPrefix will never be undefined now.

Also, I have manually verified that debugging tests (under pytest) does not fail (as in #14067) and no longer generates any entries in the "Problems" panel, whereas before (due to #14311) each test had an entry with the "Ok".

So the cause of #14067 no longer exists and #14866 is fixed.

[codecov-io]

Codecov Report

Merging #15434 (179731f) into main (22290e7) will decrease coverage by 0%.
The diff coverage is 35%.

@@          Coverage Diff           @@
##            main   #15434   +/-   ##
======================================
- Coverage     65%      65%   -1%     
======================================
  Files        561      561           
  Lines      26924    26954   +30     
  Branches    3901     3911   +10     
======================================
+ Hits       17521    17531   +10     
- Misses      8679     8695   +16     
- Partials     724      728    +4     

Impacted Files	Coverage Δ
.../client/testing/common/managers/baseTestManager.ts	44% <0%> (-1%)	⬇️
src/client/testing/common/types.ts	93% <ø> (ø)
...sting/common/services/unitTestDiagnosticService.ts	80% <50%> (-16%)	⬇️
...nts/base/locators/composite/environmentsReducer.ts	94% <0%> (-3%)	⬇️
src/client/pythonEnvironments/base/locatorUtils.ts	58% <0%> (-2%)	⬇️
...ts/base/locators/composite/environmentsResolver.ts	94% <0%> (-2%)	⬇️
src/client/common/application/notebook.ts	8% <0%> (-1%)	⬇️
src/client/pythonEnvironments/base/locator.ts	81% <0%> (ø)
src/client/common/process/rawProcessApis.ts	10% <0%> (+<1%)	⬆️
...t/jupyter/languageserver/notebookConcatDocument.ts	5% <0%> (+<1%)	⬆️
... and 1 more