Test local flow type definitions

The flow type definition tests added in #878 relied on declaring a module, which required .flowconfig to explicitly include the flow definition files, and made exports unclear. This reverts some of the structural changes to the immutable.js.flow from that PR and simplifies the .flowconfig test file to ensure tests are against the *local* copy of immutable. This uncovered a few issues with tests including a mistaken sense that types were exportes as classes and that Set intersections were causing issues.

Fixes #961