The test helper code largely predates the introduction of modern
JavaScript features and should be refactored to improve readability.
In particular callbacks and recursive function calls make the code
harder to understand and maintain.
This commit:
- replaces the callback argument with returning a promise;
- uses `const` instead of `var`;
- uses arrow functions for shorter code;
- uses template strings for shorter string formatting code;
- improves the error messages to have more details.
c9a923066b