Prompt user to configure unit tests when not already configured

[DonJayamanne]

This was originally reported by @qubitron (I thought I added an issue for this, couldn't find it, if we do, then we'll need to close this in favor that).

• Open a folder with unit tests in VS Code
• Enable Unit Tests in settings.json, but do not configure it.
• VS Code will not throw errors about unit tests not having been configured.

Note: This applies to Unit Test test framework in Python.

[qubitron]

• Add "Configure Unit Tests Command" to allow user to force configuration of unit tests if for whatever reason it can't be done automatically, add test - 2pts (Add "Configure Unit Tests" Command #4286)
• If you cancel configuration (press escape) half-way through, don't leave the settings half-configured (build up state, write everything at the end or nothing at all) - 2pts (Don't apply incomplete testing configuration. #4287)
• If you change focus out of (tab away from) VS Code, the prompt stays visible (VS Code property) - 1pts (Deal with focus when configuring testing framework. #4288)

[luabud]

When tests are not configured, they may not be discovered (if they don't follow the default config for the selected framework). When this happens, adding a button as proposed in #4318 would allow users to easily configure them, so we can wait & discuss better about prompting users when tests are left half configured.

[luabud]

Closing in favour of #4318