Our favourite tool for testing is Cypress - it's what we use for SASjs and it's user friendly and fully featured. If you have chrome it will even launch with CORS disabled for seamless API testing.
To see it in action, clone the SASjs site, add your SAS details in the [cypress.json(https://github.com/macropeople/sasjs/blob/development/cypress.json) file,
npm install and
npm run cypress.
Most likely you will only need to test you applications against a very limited set of browsers (the ones your company uses, which typically inludes IE).
For testing a wider set of browsers, the following tools are useful
- Browserstack (paid)
- https://ftp.mozilla.org/pub/firefox/releases/ (previous versions of FF)