Jest how to test GraphQL definitions

I am trying to "develop" a npm library, so I am testing them with Jest JS. I don't know what I really need to test (expect) about Jest tests.

The method buildSchema() will return a GQLSchema or GQLHTTPToolsError, and createSchema() will return a string.

This is my current test file createSchema.test.ts:

import GQLHTTPTools from '../index';
import GQLSchema from '../gqlschema';

const testTypeDef1 = `
    type Query {
        tests: [Test]
        test(id: Int): Test
    }

    type Mutation {
        createTest(name: String): Test
    }

    type Test {
        id: ID
        name: String
    }
`;

const testSchema1 = GQLHTTPTools.buildSchema(testTypeDef1);

test('create schema from single type definition', () => {
    expect(GQLHTTPTools.createSchema(testSchema1 as GQLSchema))
        .toEqual(expect.stringContaining(testTypeDef1));
});

This returns the following error:

I don't really know anything about Jest, but according to what I have read on Jest documentation, expect('my test type definition').toEqual(expect.stringContaining('type definition')) should "work", right?, I was following this Itnext guide.

What am I doing wrong?