[tslint] lint TS fixtures without types

[spalger]

In order to test that some code using our TypeScript types is flagged by the typescript compiler, it is ideal to be able to commit a fixture that includes invalid TypeScript. By explicitly excluding fixtures from our TypeScript project/tsconfig.json these files won't cause our primary TS project to fail to build, but can still be checked in tests.

This presents an issue with linting though, as we use linter rules that require type information, and therefore must compile properly.

I think we should extend the way we lint our files to use a separate TSLint instance for fixtures that does not use the type-enabled rules and only applies rules for style and such. This way we can have nicely linted TS fixtures, but also have invalid TS fixtures for testing.