Support check.v1 Test Suites

[jcalabro]

Fixes #111

This PR roughly follows the code snippet outlined here: microsoft/vscode-go#1921 (comment) to ensure that codelens is available for not just stretchr/testify, but also gopkg.in/check.v1.

The associated issue (#111) and linked issues discuss support for other testing suites as well (#228), however I did not add them as I am not familiar with them.

Taking that in to consideration, I'd love comments on:

• Is this a reasonable approach in general? It does change a few functions to by async, but the logical flow remains the same as all callers simply await as well.
• Should we spend the time to integrate a more flexible system for adding/updating testing packages? That would probably be outside the scope of this PR, but if that approach is preferred, that's pretty reasonable.

Also, this is my first contribution, and I believe I've followed the contributing guide, but please let me know if there is anything I can do to make this smoother! Thanks!

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