web-app: Add unit tests

[kimwnasptd]

/kind feature

Describe the solution you'd like
We should provide unit tests to test both the backend and the frontend of the web app kserve/kserve#1327 to ensure the distinct components of the app are working as expected

Anything else you would like to add:
I will start working on this right after the first iteration of the app's code is merged kserve/kserve#1328

/assign @kimwnasptd