Force the pytest root dir to always be the workspace root.

[ericsnowcurrently]

(for #6548)

• 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.

[codecov-io]

Codecov Report

Merging #7803 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #7803      +/-   ##
==========================================
+ Coverage   59.15%   59.16%   +<.01%     
==========================================
  Files         498      498              
  Lines       22254    22256       +2     
  Branches     3578     3578              
==========================================
+ Hits        13164    13167       +3     
+ Misses       8266     8265       -1     
  Partials      824      824

Impacted Files	Coverage Δ
src/client/testing/pytest/runner.ts	17.64% <0%> (-0.36%)	⬇️
...client/testing/pytest/services/discoveryService.ts	88.23% <100%> (+0.35%)	⬆️
src/client/common/utils/platform.ts	88.23% <0%> (+11.76%)	⬆️

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