What?

Part of: #54336

Add PHP tests for the background image block support.

Also, adjust the style injection slightly, so that if the existing style attribute ends in a semi-colon, we still inject the newly created styles.

Why?

While working on the backport for the background image block support over in WordPress/wordpress-develop#5209, I added some tests, and noticed the logic for injecting the style tag needed adjusting slightly. While in Gutenberg, inline styles don't include a trailing semi-colon, I thought it'd be good to factor it in, just in case, for the future reliability of the support and interacting with other styles.

Also, it's good to add tests for features!

How?

• Ensure that we append to the style attribute irrespective of whether or not a semi-colon already exists at the end of the current value.
• Add PHP tests for the background image support to cover injecting when no style tag exists, when a style tag exists with a single rule in it, and when there are multiple rules with a trailing semi-colon. Also check that the background image will not be output if the block does not have support for it.

Testing Instructions

1. Smoke test that the background image support still works as expected in the block editor (Add a Group block, add a background image to it, save the post, and view on the site frontend — then add some styles to the Group block, like text color, and repeat).
2. Check that tests pass, or to run npm run test:unit:php:base -- --filter WP_Block_Supports_Background_Test locally to confirm.