feat: Populate test runner with tests upon workspace open #3591

Is your feature request related to a problem? Please describe.
Before the move to the VSCode Test Runner - the test runner had a "Run All Tests" button at the top that would run and discover all tests. From that point onwards, you could run existing tests or run failed tests from the test panel.

With the move to VSCode test runner, upon opening a folder / workspace, you're greeted with "No tests have been found in this workspace" message - instead of listing of tests in the workspace that you can run.

Also - since no tests are discovered - the older functionality of "Run all tests" is not readily available to the user because the new runner doesn't run this command to discover tests.

Describe the solution you'd like
I'd like to be able to see the listing of tests upon open of the workspace. Basically, test discovery before run.

Describe alternatives you've considered
You can use the command panel to "Dart: Run All Tests" and this will discover and run the tests - adding them to the panel. This is both an extra, unclear step and does run + discover in one step instead of two separate steps.

Additional context
None.