execute = async (test, next, executeCb) => {
      const prevCpu = process.cpuUsage()
      const testFn = test.fn

      await testFn(agent)
      return after(test, next, executeCb, prevCpu)
    }
`execute` gets the CPU usage prior to the test and runs the test.
@param {Object} test configuration for the test, including the function to be tested
@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.
@returns {function} an invocation of the `after` function