I was looking for the same and here is an example of how I tested my Sales Order User Event script. Hopefully this helps

File src/FileCabinet/SuiteScripts/SalesOrder-UserEvent.js

/**

* @NApiVersion 2.1

* @NScriptType UserEventScript

*/

define([],

() => {
/**

* Defines the function definition that is executed before record is submitted.

* @param {Object} scriptContext

* @param {Record} scriptContext.newRecord - New record

* @param {Record} scriptContext.oldRecord - Old record

* @param {string} scriptContext.type - Trigger type; use values from the context.UserEventType enum

* @since 2015.2

*/

const beforeSubmit = (scriptContext) => {

if(scriptContext.type === scriptContext.UserEventType.CREATE){

scriptContext.newRecord.setValue({fieldId: 'custbody_customfield', value: Math.random()});

}

}
return {beforeSubmit}
});

File __tests__/sales-order-test.js

import record from 'N/record';

import Record from 'N/record/instance';

import SalesOrderUserEvent from 'SuiteScripts/SalesOrder-UserEvent';
jest.mock('N/record');

jest.mock('N/record/instance');
beforeEach(() => {

jest.clearAllMocks();

});
describe('Sales Order Test', () => {

it('should add a random number to the sales order on create', () => {

// given

const salesOrderId = 1352;

record.load.mockReturnValue(Record);

Record.save.mockReturnValue(salesOrderId);
const scriptContext = {

type: 'create',

newRecord: Record,

UserEventType: {

CREATE: 'create',

EDIT: 'edit',

}

};
// when

SalesOrderUserEvent.beforeSubmit(scriptContext);
// then

expect(Record.setValue).toHaveBeenCalledWith({fieldId: 'custbody_customfield', value: expect.anything()});

});
it('should not add a random number to the sales order on edit', () => {

// given

const salesOrderId = 1352;

record.load.mockReturnValue(Record);

Record.save.mockReturnValue(salesOrderId);
const scriptContext = {

type: 'edit',

newRecord: Record,

UserEventType: {

CREATE: 'create',

EDIT: 'edit',

}

};
// when

SalesOrderUserEvent.beforeSubmit(scriptContext);
// then

expect(Record.setValue).not.toHaveBeenCalled();

});

});