Move _generate_expected_image out of _generate_test

These two functions serves different purpose which are better kept
separate. _generate_expected_image in particular is part of the test
generator setup as it requires updating the params dictionary.

Bug: 330885075
Change-Id: Iaac14adcc14e860c80f6197a978d171297b2699d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5374660


Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282134}