In Imagick tests, instantiate `WP_Image_Editor_Imagick` directly instead of calling `wp_get_image_editor()`.

This is for greater consistency with other image editor tests.

Props voldemortensen.
Fixes #30820.

git-svn-id: https://develop.svn.wordpress.org/trunk@30990 602fd350-edb4-49c9-b593-d223f7449a82