Expose TestModuleConfig interface

**Is your feature request related to a problem? Please describe.**

In tests, I may want to define a variable of type `TestModuleConfig` scoped for the entire test suite. It is not possible to get compile errors at the declaration site because `TestModuleConfig` is not exposed. Further, if I want to modify that object later, I won't receive compile time errors at that line of code.

**Describe the solution you'd like**

Export this type to users of `graphql-modules` https://github.com/Urigo/graphql-modules/blob/75552e6775202a8c8a69d3aa587033d9c0987da2/packages/graphql-modules/src/testing/test-module.ts#L29-L38

**Describe alternatives you've considered**

Copy/pasta of the type into a project or each test file is not ideal, especially if the type changes on this project.

**Additional context**
If the type is exposed, users would be able to do this.
```ts
// using something like Jest
describe('my test', () => {
let config: TestModuleConfig;

beforeEach(() => {
config = { ...baseConfigStuff };
});

test('first test', () => {
const app = testkit.mockModule(someModule, config);
});

test('second test which modifies config', () => {
config.typeDefs = 'wrong type'; // GET AN ERROR RIGHT HERE'
const app = testkit.mockModule(someModule, config);
});
```

Perhaps there are other types worth exporting as well, but this is one I've come across thus far.