Export MockResolverContext type on relay test utils

Summary: We are exporting `MockResolverContext` so that the type can be used explicitly in test utilities and make it easier to reuse testing logic .

Differential Revision: D67139589

fbshipit-source-id: ca48764b2665cb4a6579f539504ddc187e765ddd

Author: Nico Robin

packages/relay-test-utils/RelayMockPayloadGenerator.js

@@ -71,7 +71,7 @@ type Traversable = {
   +args: ?{[string]: mixed, ...},
 };
 type MockData = {[string]: mixed, ...};
-type MockResolverContext = {
+export type MockResolverContext = {
   +parentType: ?string,
   +name: ?string,
   +alias: ?string,