sfdx-lwc-jest utility for testing async functionality

jest.setup.js

global.waitFor = (expectationFunction) => {
    const MAX_WAIT = 5000;
    const POLL_INTERVAL = 100;
    return new Promise((resolve, reject) => {
        const startTime = Date.now();
        let error;
        const interval = setInterval(() => {
            try {
                if (Date.now() - startTime < MAX_WAIT) {
                    expectationFunction();
                    clearInterval(interval);
                    resolve();
                } else {
                    clearInterval(interval);
                    reject(error);
                }
            } catch (e) {
                error = e.matcherResult;
            }
        }, POLL_INTERVAL);
    });
};
// in your test method call waitFor like this
const button = element.shadowRoot.querySelector('button');
button.dispatchEvent(new CustomEvent('click')); // calls an async handler

await waitFor(() => { expect(createUserKey.mock.calls.length).toBe(1) }); // wait for you mock to be called within the async handler