Add workspace root to test explorer tree for multi-root scenario

[eleanorjboyd]

Comes from this discussion: #22125 (reply in thread).

The issue is that in multi-root workspace where the "--rootdir", "${workspaceFolder}/tests" arg exists or the "python.testing.cwd": "${workspaceFolder}/tests", exists then the root is not the root of the multiroot workspace and therefore the root node on the test explorer is the defined rootdir instead of the root of the workspace. This leads to multiple workspace where it is has a tests folder per workspace to then just be displayed as such:

• tests
  • test_1
  • test_2
  • test_3
• tests
  • tests_a
  • tests_b
  • tests_c

the desired test explorer view would instead be:

• workspace 1
  • tests
    • test_1
    • test_2
    • test_3
• workspace 2
  • tests
    • tests_a
    • tests_b
    • tests_c

[github-actions]

Thanks for the feature request! We are going to give the community 60 days from when this issue was created to provide 7 👍 upvotes on the opening comment to gauge general interest in this idea. If there's enough upvotes then we will consider this feature request in our future planning. If there's unfortunately not enough upvotes then we will close this issue.

[brettcannon]

Thank you for submitting your feature request and everyone who considered it! Unfortunately, this issue did not receive enough votes over the allotted time, and so we are closing the issue.