Auto-detect src and/or test directories

[brettcannon]

Some people keep their source in a src directory and some people keep their tests in test. This seems to up often enough that we should either detect this and automatically do something or warn people they should create a .env file that manipulates their PYTHONPATH to include the appropriate directories.

[DonJayamanne]

and some people keep their tests in test.

We already have some code that looks for directories named test (or similar) and we display a prompt asking the user whether they'd like to configure VS Code to discover and run the tests.

[DonJayamanne]

We might want to update our documentation too (see #1328), other IDEs do the same.
Besides that's how python works (using PYTHONPATH)

[DonJayamanne]

We already have some code that looks for directories named test (or similar) and we display a prompt asking the user whether they'd like to configure VS Code to discover and run the tests.

I take that back, the auto detection of tests was take out of the extension 99ff84c
I guess this was removed as users found the prompts too annoying.

[DonJayamanne]

If we do go back with the auto-prompt, we might want to make the prompt more subtle. As it is we have a linter prompt. I guess it would be a review of the extension startup (setup) experience.