This CL is Step 3 of the singleton migration for WorkspaceDiffImpl.
It migrates WorkspaceDiff.test.ts to use TestUniverse and standard
describe blocks with setup hooks.

TAG=agy
CONV=26ac8703-241b-4c79-8df7-090342f38096
Bug: 493763857
Change-Id: I253774db1eb0dec687b2749cd9c0c39e8aec35ca
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8063784
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Auto-Submit: Kim-Anh Tran <kimanh@chromium.org>