Add webapp e2e tests for model training

[hleepfl]

There are currently very few webapp e2e tests that check whether tasks are working.

It would be nice to have

• tests covering local training for default tasks (at least one test per data type, and one for decentralized case)
• tests covering collaborative training for default tasks

Webapp E2E testing for collaborative tasks is harder as it requires running several test browsers at the same time. Will require some research to know if it's feasible.