Use salsa to test against existing tests in source

[unional]

Originated from #59

The idea is to leverage test cases already written by the package author to test against the typings.

The challenge we face are as follows:

• Currently salsa does not support cli access:
  Validate JS file against typings? microsoft/TypeScript#7661
• The test harness used by the package author and the way they use it can varies
• Some modification of the tests are needed
  Add source tests typed-typings/npm-chai#7 (comment)

[unional]

Um.....should I put this under generator-typings instead?

[blakeembrey]

This is fine, also typings/typings has more visibility too for discussions.

Once --allowJs works programmatically, I think perhaps the other pieces will fall into place. --allowJs should enable following CommonJS resolution and it does fall back to definition support. So typings bundle + tsc --allowJs + test runner should work (fingers crossed).

[unional]

This is more of an action-item than discussion, so I think it is ok to leave it here. 😄