The test helper code largely predates the introduction of modern
JavaScript features and should be refactored to improve readability.
In particular callbacks 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;
- uses `Array.includes` for shorter response code checking code.
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