Fix the unit tests for Imagick alpha.

These tests were written to use the `assertImageAtAlpha()` helper method, whose
internals were specific to GD. As a result, the tests could not pass. This
changeset introduces GD- and Imagick-specific versions of this method.

Props voldemortensen.
Fixes #24871.

git-svn-id: https://develop.svn.wordpress.org/trunk@30549 602fd350-edb4-49c9-b593-d223f7449a82