Relay: Use GraphQL RFC for Relay Tests

Summary: Makes a couple changes that allow us to use the GraphQL RFC tests (that currently exist on GitHub) internally in `static_upstream/relay`.

 - Change Jest to use `babel-relay-plugin` and `testschema.json` for tests in `static_upstream/relay`.
 - Use the OSS forks when running tests without `RelayTestUtils.unmockRelayForFB()`.

Reviewed By: @wincent

Differential Revision: D2414174

src/tools/__mocks__/RelayTestUtils.js

@@ -455,10 +455,11 @@ var RelayTestUtils = {
 
   unmockRelayForFB() {
     RelayTestUtils.unmockRelay();
-    global.__RELAYFB__ = true;
+    global.__RELAYOSS__ = false;
   },
 
   unmockRelay() {
+    global.__RELAYOSS__ = true;
     jest
       // Utilities
       .dontMock('areEqual')