function mustCall(func) {
		callCounts.set(func, 0);
		return function Wrapper(...args) {
			callCounts.set(func, callCounts.get(func) + 1);

			return func.call(this, ...args);
		};
	}
Asserts that a given function is called at least once during a test
@param {Function} func The function that must be called at least once
@returns {Function} A wrapper around the same function