Refactor AI Assistance unit tests

This CL removes boilerplate from the unit tests.

Bug: none
Change-Id: I22eef57a160be558b806b0087d959fe649a17833
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6259395
Reviewed-by: Nikolay Vitkov <[email protected]>
Commit-Queue: Alex Rudenko <[email protected]>
Reviewed-by: Ergün Erdoğmuş <[email protected]>

Author: OrKoN