yarn-puppeteer does not build due to failing tests

## Affected builder image
`gcr.io/cloud-builders-community/yarn-puppeteer`

## Expected Behavior
Test steps should pass in cloud build

## Actual Behavior
Jest takes too long to exit and thus exits non-zero:

```
Already have image: gcr.io/tara-frontend/yarn-puppeteer:node-9.11.1
yarn run v1.5.1
warning ../../../package.json: No license field
$ jest --config=jest-puppeteer.js

Jest dev-server output:
warning ../../../package.json: No license field
[Jest Dev server] $ node .
[Jest Dev server] listening on port 3000.

PASS tests/index.test.js
index.js
✓ renders / (18ms)

Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 0.821s
Ran all test suites.
Jest did not exit one second after the test run has completed.

This usually means that there are asynchronous operations that weren't stopped in your tests. Consider running Jest with `--detectOpenHandles` to troubleshoot this issue.
```

## Steps to Reproduce the Problem

1. `cd yarn-puppeteer`
2. `gcloud builds submit . --config=cloudbuild.yaml`

## Additional Info
Manually running the tests locally works fine, and commenting out the test steps allows a functional image to be deployed.