after = async (test, next, executeCb, prevCpu) => {
      // The cpu delta is reported in microseconds, so we turn them into
      // milliseconds
      const delta = process.cpuUsage(prevCpu).user / 1000
      // Despite an effort to reduce callbacks in this test class, some remain, like this one:
      const afterCallback = () => next(null, delta)

      if (typeof test.after === 'function') {
        test.after()
      }

      if (typeof executeCb === 'function') {
        return executeCb(afterCallback)
      }

      return afterCallback()
    }
Function that calculates CPU usage after a test, and calls any defined after/callback
@param {Object} test Object defining the current test configuration
@param {function} next anonymous function defined in the `startTest` for loop;
  its second parameter is the delta between the current CPU usage and the previous CPU usage.
@param {?function} [executeCb] If the test is run in a transaction, `executeCb` is defined,
  and will run after any user-defined `after` function. If `executeCb` is defined, the result of
  that is the `after` function's return value.
@param {{user: number, system: number}} prevCpu Output of process.cpuUsage() in the previous test
@returns {function} an invocation of `executeCb`, if defined, or `next(null, delta)`