Add tools to run the tests locally

To run the tests:

- From the menu, select Tools > Build Systems > Run

This activates the build systems specified in the .sublime-project
file.

- To run all tests:

  From the command palette, select Build: Dart - Test (All)

- To run a test*.py file only:

  * Activate the corresponding file in ST.
  * From the command palette, select Build: Dart - Test (This File Only)