[test] Update user agent

[TobiGr]

• [x] I carefully read the contribution guidelines and agree to them.
• [x] I have tested the API against NewPipe.
• [x] I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.

This updates the user agent used for testing purposes to a modern version.

All tests passing the real tests also pass the new mock tests.

[TobiGr]

~~All tests passing the real tests also pass the new mock tests.~~