Media: Improve support for opting out of lazy-loading for template images.

With this changeset, in addition to the already present `wp_lazy_loading_enabled` filter, developers can now opt out of lazy-loading template images via `wp_get_attachment_image()` by passing a `loading` attribute with boolean value `false`. This can be used e.g. by theme developers on images which are very likely to be in the initial viewport.

This changeset also improves related test coverage.

Props adamsilverstein, azaozz, joemcgill, johnbillion.
See #50425, #44427.

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git-svn-id: https://core.svn.wordpress.org/trunk@48041 1a063a9b-81f0-0310-95a4-ce76da25c4cd