test-gegl: Assert the bounding box when zooming GeglBuffers

While the SHA256 checksum is the ultimate source of truth when
comparing GeglBuffers, the checksum values provide very little insight
into why the buffers are different. Therefore, it's better to assert
the more human understandable properties of the GeglBuffer before
looking at the checksum.

!99
3 jobs for wip/rishi/tests-gegl-assert-bbox in 14 minutes and 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #220993
flatpak

00:13:40

 
  Review
passed #220994
review

00:00:35

passed #220995
allowed to fail manual
stop_review

00:00:25