tests: Don't check pixels outside actor allocation

tests: Don't check pixels outside actor allocation

The actor-shader-effect test actors are 50px wide, but we check the 51st
pixel. This went along undetected until "clutter: Avoid rounding
compensation when invalidating 2D actors" because the paint volumes were
made slightly bigger and the shaders paint all over them (I guess nobody
noticed those actors being actually ~52px wide).

Update the test to check the middle of the opposite edge, so we keep neatly
rounded numbers.

(cherry picked from commit 1d73533f786b8f83fcffae7e5f5965d3b1f4e57a)

Applied-upstream: 3.30.3, commit:5e4cb54bb5ced1a5829fb903fe1729f5da248b0c

Gbp-Pq: Name tests-Don-t-check-pixels-outside-actor-allocation.patch