Support multi-workspace folders in test explorer

[DonJayamanne]

For #4268

• 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!)
• Has sufficient logging.
• [n/a] Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• [n/a] Test plan is updated as appropriate
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)
• [n/a] The wiki is updated with any design decisions/details.

[codecov]

Codecov Report

Merging #4709 into master will increase coverage by 1%.
The diff coverage is 62%.

@@          Coverage Diff           @@
##           master   #4709   +/-   ##
======================================
+ Coverage      77%     77%   +1%     
======================================
  Files         447     447           
  Lines       21454   21472   +18     
  Branches     3523    3527    +4     
======================================
+ Hits        16424   16441   +17     
- Misses       5026    5027    +1     
  Partials        4       4

Flag	Coverage Δ
#Linux	66% <53%> (+1%)	⬆️
#Windows	66% <53%> (+1%)	⬆️
#macOS	66% <53%> (+1%)	⬆️

[karrtikr]

I was expecting that adding a new workspace folder, should autoDiscover tests for the new folder. It's not happening right now, is it the right behavior?

[karrtikr]

Please address the comments.