no-unused-modules: add flow tests and enhance typescript tests

[nicolashenry]

Try to reproduce #1564 :

• Add tests for flow type imports
• Remove typescript tests using "babel-eslint" (because resulting types seems to be flow types)
• Add some missing cases in typescript tests
• Add some tests for typescript type imports

[coveralls]

Coverage remained the same at 97.906% when pulling a022baf on nicolashenry:feature/enhance-tests into fef718c on benmosher:master.

[ljharb]

For future reference, just for running tests, you can enable CI on your fork and push to that :-)

Let's keep this open so I can sync it up, if needed, with a different fix PR (best, of course, is if this can be the fix PR!)

[ljharb]

@nicolashenry it'd be great if you could rebase this on latest master :-)

[nicolashenry]

I think you already included it in 5ade156 (it seems) so I think I can close it.