Add unit tests for pipenv/Pipfile detection

[brettcannon]

It would seem there are either no unit tests or they are "broken" from the perspective of what they are testing since the Pipfile/pipfile fix didn't fail either.

[MikhailArkhipov]

https://github.com/Microsoft/vscode-python/blob/master/src/test/install/channelManager.channels.test.ts

[DonJayamanne]

We do have some tests, but we've missed some areas (its a coverage issue).
https://codecov.io/gh/Microsoft/vscode-python/src/master/src/client/interpreter/locators/services/pipEnvService.ts#L75

Question is should we aim for a higher coverage?

[MikhailArkhipov]

I think in this particular case factor is a combination of specific OS and use of mocks. Test needs to run on proper platform with real workspace on disk rather than via simulated file system mock.