Media: Some documentation and test improvements for WebP support:

* Document that WebP constants are only defined in PHP 7.1+.
* Correct the `$filename` parameter type in `wp_get_webp_info()`.
* Use a consistent message when skipping tests due to the lack of WebP support.
* Remove unnecessary `else` branches after `markTestSkipped()`.
* Replace `assertEquals()` with more appropriate assertions.

Follow-up to [50810].

See #35725.

git-svn-id: https://develop.svn.wordpress.org/trunk@50814 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -3079,9 +3079,11 @@ function wp_get_image_mime( $file ) {
 			return false;
 		}
 
-		// Add WebP fallback detection when image library doesn't support WebP.
-		// Note: detection values come from LibWebP, see
-		// https://github.com/webmproject/libwebp/blob/master/imageio/image_dec.c#L30
+		/*
+		 * Add WebP fallback detection when image library doesn't support WebP.
+		 * Note: detection values come from LibWebP, see
+		 * https://github.com/webmproject/libwebp/blob/master/imageio/image_dec.c#L30
+		 */
 		$magic = bin2hex( $magic );
 		if (
 			// RIFF.