Warn the user when testing configuration is not complete

[DonJayamanne]

On behalf of @qubitron

Repo: github.com/qubitron/hello-stackoverflow

Scenario 1

• Enable unit tests (do not configure the args)
• Running and discovering tests fail

Suggestion: Check if args are empty, if they are, then display a prompt to configure test framework

Other errors

• Testing with PyTest failed
• Debug adapter failed regularly leaving extension in a bad state (requiring restart of VS Code)
  • Status bar had a message running tests (this never went away)
  • Debugger bar appeared on the top (clicking stop did nothing)
  • Debug adapter failed with usual message Debug adapter failed unexpectedly
• Deleting Experimental Debugger configuration settings from launch.json fixed a lot of the issues

Todo:

• Improve error handling and logging (diagnosing)
• User should never have to reload VS Code
  * Recover from a state where debugger has a message running tests

[DonJayamanne]

This issue depends on #1068
@brettcannon cc