Imagick Preserve Alpha Unit Tests Broken

[bpetty]

I'm still not entirely sure why the following two tests aren't working correctly, however, alpha in the resulting files they are testing for do in fact preserve alpha correctly.

1. Tests_Image_Editor_Imagick::test_image_preserves_alpha_on_resize
2. Tests_Image_Editor_Imagick::test_image_preserves_alpha

It might be helpful though to at least update the code in WP_Image_UnitTestCase::assertImageAlphaAtPoint() to implement this in each editor engine so the Imagick tests actually use Imagick to check for alpha instead of GD, and in the process, that might resolve this.

[markoheijnen]

I think it's broken in some versions of imagemagick but I'm not sure. Currently I do think some stuff needs to be rewritten but I haven't had the change yet to do so.

[boonebgorges]

In 30549:

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.