forest: Fix tests with forest enabled (4/N)

Fixes desks and wallpaper tests. For desks, the size of the expanded
desks bar in overview is larger. For wallpaper, there is a new solid
color, wallpaper under layer that should be considered in tests.

I opted not to add a new parameter since this is only temporary, and I
didn't want to add more tests. Especially with the desks tests, adding
a new boolean parameter would mean 8 more variations per test.

Test: none
Bug: b:338414459
Change-Id: I818fd875c7d31942ab4d01e1813c8d2ede9bfb83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5519278
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1297172}