If you use the Chrome browser, then you already have some accessibility testing tools built in.
Open the Developer Tools, and click into the Lighthouse tab. This allows you to audit an page you are currently using and check for things like Performance, Accessibility, Best Practices, and SEO.
You won’t be able to change everything it recommends doing, but you should be able to make some improvements.
Hope this helps.
If you’re referring to testing of your application, please have a look here: https://www.mendix.com/evaluation-guide/app-lifecycle/test-automation-quality-assurance/
If you’re referring to security testing: Burpsuite for example can be perfectly used for Mendix applications. We do security checks all the time on the Mendix solution.
I hope this answers your question.